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Elements of 
ECOLOGY 



Elements of 
ECOLOGY 

GEORGE L. CLARKE 

Harvard University 

and 
Woods Hole Oceanographic Institution 



JOHN WILEY & SONS, INC. 
New Yorfc 

CHAPMAN & HALL, LTD. 
London 



COPYRIGHT, 1954 

BY 
JOHN WILEY & SONS, INC. 



All Rights Reserved 

This book or any part thereof must not 
be reproduced in any form without 
the written permission of the publisher. 



THIRD PRINTING. APRIL. 1959 



Contribution No. 704 from the Woods Hole Oceanographic Institution 
Library of Congress Catalog Card Number: 54-12205 

PRINTED IN THE UNITED STATES OF AMERICA 



To 
M. S. C. 



Preface 



"Live Alone and Like It" is a slogan that no living thing can adopt. 
Every plant and animal is subject to both the living and the non-living 
influences of its surroundings. Every organism depends upon its 
environment to supply it with vital materials and energy. Every 
living being must share its world with members of its own species and 
with members of other species be they friend or foe. 

Man is no exception. Man is surrounded by many kinds of living 
things, and he must derive his needs from the world around him. Man 
must learn to live in adjustment with his fellow men, and with the 
plants and animals of his environment, and to use his natural resources 
judiciously, or he will be exterminated. 

Obviously, then, the interrelations of the organism and its environ- 
ment are crucial; and ecology, which is the study of these interrela- 
tions, is of great significance. Yet few books are available which deal 
with the general principles of the whole subject. Most of the books 
in the field of ecology treat primarily either "plant ecology" or "animal 
ecology." But animals cannot get along without plants, and plants 
are almost always vitally influenced by animals. Man is dependent 
upon both. 

Accounts of the flora and fauna of various regions and descriptions 
of a variety of habitats have long been available. Early ecologists 
have investigated the effects of environmental factors on the activities 
of living things. But not until relatively recently have we realized 
that plants and animals also react on their surroundings in such a way 
that they form a reciprocating and integrated system with their 
environment. The modern ecologist focuses his attention on the 
dynamic interplay of the forces in the living community. 

The purpose of this book is to bring together in one place and in a 
simple way the elements of ecology with special emphasis on the 
modern viewpoint of the science. It is desired to stress the unity of 
ecology and the necessity for including the influence of both plants 
and animals as well as the physical forces as part of the environment. 
The subject is old, but efforts to crystallize the basic principles in any 
exact way are relatively new. The potential scope of ecology is very 

vii 



viii Preface 

great, and no attempt at any exhaustive treatment of the subject is 
made here. Rather, this book presents an introductory account of the 
fundamental relationships of ecology upon which an understanding 
of the life of the individual and of the community depends. Since 
many of these relationships are seen in a relatively simple form in the 
marine environment, many illustrations are taken from marine habitats. 

The book is written primarily for students in ecology. It is also 
intended as background material for those interested in conservation, 
forestry, agriculture, fisheries, wildlife management, and other 
branches of "applied ecology," -The general biology student should 
also derive value from the book since a knowledge of the principles 
governing the real lives of animals and plants in nature is a vital 
backdrop for problems in physiology, morphology, genetics, and 
evolution. Furthermore, it must be remembered that every one of us 
is an organism with an environment and that each of us forms part of 
the environment for other living beings. The general reader, as well 
as the biology student, is interested to know how the world of life 
works. Populations of men follow many of the same laws as popula- 
tions of microbes. The need for a knowledge of the proper adjust- 
ment among men and between man and his natural environment was 
never greater than it is today. 

The material of the book is drawn in large part from a course in 
ecology given by the author at Harvard University. Many ideas and 
illustrations have been derived from discussions with colleagues at 
Harvard University, the Woods Hole Oceanographic Institution, and 
elsewhere, and acknowledgment is made of this material. I am also 
indebted to W. C. Allee, S. A. Cain, J. T. Curtis, R. W. Dexter, W. H. 
Drury, E. H. Graham, D. R. Griffin, H. C. Hanson, A. G. Huntsman, 
S. C. Kendeigh, B. H. Ketchum, P. C. Lemon, D. Merriman, R. S. 
Miller, E. W. Moore, E. T. Moul, O. Park, T. Park, H. M. Raup, 
A. C. Redfield, F. A. Richards, G. A. Riley, J. H. Ryther, P. S. Sears, 
J. G. Steel, H. C. Stetson, and J. M. Teal for reading parts of the 
manuscript and offering helpful criticisms. I am particularly grateful 
to Marion S. Clarke, Donald Kennedy, and J. E. G. Raymont for 
assistance in preparing the manuscript and for many valuable 
suggestions. 

GEORGE L. CLARKE 

August, 1954 



Contents 



Chapter 1 VIEWPOINT OF MODERN ECOLOGY 1 

The Meaning of Ecology 2 

The Meaning of Environment . 3 

The Critical Environment 5 

The Development of Ecology 13 

The Ecology of Plants and Animals 13' 

The Ecology of Habitats and of Individuals 14 

The Ecology of Populations and of Communities 14 

The Ecological Complex 16 

The Scope of Ecology 18 

Approach to the Study of Ecology 20 

Chapter 2 THE MEDIUM 23 

Contrasting Qualities of Air and Water 24 

Pressure 27 

Pressure Reduction with Altitude 28 

Pressure Increase with Depth 29 

Support and Resistance to Motion 32 

Effects on Structure and Size ,32 

Effects on Locomotion through Medium 34 

Passage of Medium through Organism 35 

Existence of Plankton 36 

Transportation by Medium 42 

Sessile Existence 44 

Distribution by Medium 45 

Transport by Air 45 

Transport by Water 48 

Harmful Transport 52 

Abrasive Action of Medium 53 

Chapter 3 - THE SUBSTRATUM 59 

Significance of the Substratum 59 

Needs Provided by the Substratum 59 

Attainment of the Substratum 60 

Reactions to the Substratum 61 

The Variety of Substrata 62 

Rock, Sand, and Mud in Aquatic Environment 66 

Influence of the Aquatic Substrata 66 

Breakdown of the Substratum 69 

ix 



x Contents 

Build-Up of the Substratum 69 

Rock, Sand, and Soil in Terrestrial Environment 71 

Influence of the Land Substrata 71 

Land Surfaces and Animals 72 

Soil in Its Action on Plants 73 

Action of Organisms on Soil 76 

Abundance of Organisms in Soil 76 

Soil Formation 77 

Humus and the Colloidal Complex 79 

The Soil Profile 80 

Soil-group Divisions 84 

Chapter 4 - WATER 90 

Water Problem in the Aquatic Environment 91 

Composition of Natural Waters 91 

Methods of Meeting Osmotic Problem . . ........ 95 

Limiting Effects of Salinity 97 

Amphibious Situations 99 

Swamps and Temporary Pools 99 

Tidal Zone 101 

Water Problem in the Terrestrial Environment 107 

Occurrence of Water in Land Environment 109 

Moisture in the Soil Ill 

Moisture in the Air 112 

Microclimates 117 

Meeting Water Problem on Land 119 

Influence of Moisture on Growth and Distribution . . . . . . 121 

Chapter 5 TEMPERATURE 129 

Distribution of Temperature 129 

Extremes of Temperature and of Tolerance 129 

Changes in Temperature 131 

Changes in Time 131 

Horizontal Changes 134 

Vertical Changes 134 

Seasonal Changes in Vertical Temperature Structure .... 138 

Biological Action of Temperature . 141 

Extreme Temperature 142 

Minimum Temperatures 142 

Maximum Temperatures 143 

Methods of Meeting Temperature Extremes 144 

Morphological and Physiological Adaptations 144 

Thermal Migrations 148 

Action within Effective Range 152 

Effect of Temperature on Biological Rates 152 

Optimum Temperature 155 

Other Effects of Temperature 156 

Action of Temperature on Distribution 159 

Mode of Temperature Limitation 159 

Control by Extremes 160 



Contents xi 

Control by Need for Minimum Amount of Heat 164 

Control by Need for Chilling 168 

Results of Temperature Limitation 172 

. Special Cases of Common Boundaries 172 

Frost Line 173 

Tree Line 173 

Life Zones 175 

Temperature and Moisture Acting Together 181 

Chapter 6 LIGHT 185 

Distribution of Light 186 

Light on Land 186 

Spectral Composition 186 

Intensity of Light 187 

Duration and Amount of Light 188 

Light in Water 191 

Extinction and Modification of Light 191 

Changes in Transparency 197 

Biological Effects of Light 198 

General Effects 198 

Protective Coloration 199 

Activity and Vision 202 

Photokinesis 202 

Vision 202 

Bioluminescence 204 

Orientation 207 

Periodicity 216 

Diurnal Periodicity 216 

Lunar Periodicity 219 

Seasonal Periodicity 224 

Ultraviolet Light 230 

Ecological Aspects of Photosynthesis 233 

Land Plants 233 

Aquatic Plants 235 

Chapter 7 OXYGEN AND CARBON DIOXIDE 242 

Oxygen 242 

Availability of Oxygen 243 

Terrestrial Environment 243 

Aquatic Environment 244 

Oxidation-Reduction Potential 251 

Effects of Oxygen Availability 253 

Terrestrial Environment 253 

Aquatic Environment 255 

Carbon Dioxide 259 

Carbon Dioxide in Terrestrial Environment 260 

Carbon Dioxide in Aquatic Environment 262 

Reactions of Carbon Dioxide in Water 262 

Ecological Effects of Carbon Dioxide 265 



xii Contents 

Hydrogen Ion Concentration 268 

Calcium Carbonate 270 

Chapter 8 NUTRIENTS 277 

Nutrients and the Environment 277 

Modes of Nutrition 278 

Influence of Nutrients on Green Plants 280 

Nutrients Required 280 

Law of the Minimum 281 

Limitation by Nutrients in Nature 283 

Influence of Nutrients on Animals 292 

Decomposition and Regeneration 297 

Processes of Decomposition and Transformation 298 

Place of Decomposition 300 

Stagnation in Cycles 303 

Regeneration 304 

Rate of Regeneration 305 

Ratio of Regenerated Materials 306 

Chapter 9 - RELATIONS WITHIN THE SPECIES 309 

Origin of Groups 309 

Reproduction 309 

Passive Transport 311 

Active Locomotion 312 

Common Orientation 312 

Mutual Attraction . 314 

Effects of Increased Numbers 316 

Harmful Effects 317 

Beneficial Effects 322 

Protection 322 

Influence on Reproduction 323 

Division of Labor 327 

Population Development 333 

Natality and Mortality 333 

, Biotic Potential and Environmental Resistance 334 

Form of Population Growth ^ 336 

Logistic Curve . < 337 

Equilibrium and Fluctuation 340 

Optimal Yield 344 

Spatial' Relations of Populations 348 

Space Requirements 348 

Home range and Territory 350 

Homing 353 

Return Migration 355 

Emigration 357 

Chapter 10 RELATIONS BETWEEN SPECIES 362 

Symbiosis 364 

Mutualism 364 

Mutualism with Continuous Contact 364 

Mutualism without Continuous Contact 370 



Contents xiii 

Commensalism 374 

Commensalism with Continuous Contact 374 

Commensalism without Continuous Contact 377 

Antagonism 381 

Antibiosis 383 

Exploitation 385 

Parasitism 386 

Predation . . 392 

Competition . . 396 

Chapter 11 THE COMMUNITY 401 

Community Concept . 401 

Community Dominance . 408 

Ecotone . 410 

Community Composition 114 

Stratification of the Community 120 

Chapter 12 SUCCESSION AND FLUCTUATION 125 

Ecological Succession 125 

Dispersal and Invasion 127 

Barriers 428 

Ecesis 429 

Succession and Climax 430 

Types of Succession 432 

Primary Succession 432 

Secondary Succession 435 

Convergence 438 

Succession in Special Habitats 438 

Modification of Succession 444 

Community Classification 447 

Community Type 447 

The Biome 448 

Fluctuation within the Community 451 

Irruptive Fluctuation 453 

Year-Class Analysis 454 

Cyclic Fluctuation 457 

Causes of Fluctuation 458 

Origin of Cycles 460 

Chapter 13 DYNAMICS OF THE ECOSYSTEM 465 

Fundamental Operation 465 

Principal Steps and Components 466 

Niches 468 

Trophic Levels and Relations 469 

Ecological Cycle in the Ocean 473 

Producers 474 

Consumers 476 

Herbivores 476 

Carnivores 479 

Decomposers and transformers 481 



xiv Contents 

Productivity of the Ecosystem 481 

Concepts of Productivity 482 

Standing crop 483 

Relation to Population Growth 483 

Determination of Equilibrium Level 485 

Regional differences 487 

Material Removed 489 

Production rate 493 

Turnover 496 

Efficiencies 497 

Productivity of Land and Water 500 

Conclusion 503 

References 505 

Index 523 



Viewpoint of 
Modern Ecology 



Every living thing is surrounded by materials and forces which con- 
stitute its environment and from which it must derive its needs. 
Contact with the environment is inescapable. Protoplasm, the essen- 
tial constituent of an organism, is a dynamic substance, requiring a 
continuous exchange of energy and materials. In order to remain 
alive the organism, too, must secure energy and materials, and these 
can be procured only by exchange with the outside world. Thus the 
animal or plant cannot live completely sealed in an impervious skin or 
shell but requires from its surroundings: (1) a supply of energy; (2) a 
supply of materials; (3) a removal of waste products. The exchanges 
that the organism has with its environment may be thought of as an 
"external physiology," and they are just as essential as its internal 
physiological adjustments. 

The supplying of the vital needs of the organism is by no means 
the only action of the environment. Since the animal or plant must 
leave its borders open to foreign trade, as it were, the possibility 
exists that harmful materials will enter the organism or destructive 
Influences act upon it. For example, algae in a river must be suffi- 
ciently permeable to take in the water needed for their metabolism, 
but, if they drift into the sea, the higher salinity will cause a fatal loss 
of water from their tissues. Animals in the environment of these 
algae tend to add materials to the water which serve as plant nutri- 
ents, but these same animals may also feed upon the algae and 
eventually destroy the population. 

In order for the life of an organism to continue, the environment 
must be satisfactory on two counts: (1) it must provide the minimum 
requirements for life; (2) it must contain no influence incompatible 
with life. If there is too little water, as in the Sahara Desert, or too 
little oxygen, as at the top of Mt. Everest, or if nutrients are not avail- 
able, as on rocky plateaux, animals or plants cannot obtain their 

1 



2 Viewpoint of Modern Ecology 

minimum needs. In other situations an ample supply of food and 
light may be found, but the temperature may be so high as to exclude 
all life. This is true in some hot springs. Certain soils of southwestern 
United States are completely devoid of vegetation because of the 
excessive amounts of salts or "alkali." Death Valley on the Dieng 
Plateau in Java is populated with plants but no animal life can exist 
because of high concentrations of carbon dioxide issuing from subter- 
ranean crevices. Such influences are prohibitory to life and others 
are harmful without being lethal. In a complex natural situation it 
is not always easy to discover the particular role each factor plays, 
but we can be certain that in every situation dealings with the environ- 
ment are inescapable. 

THE MEANING OF ECOLOGY 

The study of these interrelations of plants and animals with their 
environment constitutes the science of ecology. The environment in- 
cludes the influences of other plants and animals present as well as 
those of the physical features. In order to investigate the exchanges 
and interdependencies involved, it is necessary to have a knowledge 
both of the organisms themselves and of the environments inhabited 
by them. The ecologist must know the material with which he works. 
He must have a grasp of the classification and the structure of plants 
and animals, and he must understand what makes them tick. At 
the same time the ecologist must be thoroughly aware of the nature 
of the environment both living and non-living. He must be familiar 
with the different types of terrain on land and with the different 
qualities of water in the ocean, lakes, and rivers. He must appreciate 
the special environmental conditions provided by the various kinds 
of vegetation. He must have a knowledge of the circulation of water 
and air and of the dynamic processes going on in the soil. But the 
taxonomy, morphology, and physiology of organisms and the physi- 
ography of land and sea, although providing a necessary background, 
do not form the central core of ecology. The ecologist focuses his 
attention primarily on the interrelationships between the organism 
and its environment: 

Ecology 

/4-\ 

Organism 4-V Environment 

The word "ecology" comes from the Greek "oikos," meaning "home" 
or "estate" hence, ecology is the study of the home, or how the house- 



The Meaning of Environment 3 

hold of nature is kept in order. Interestingly enough, although 
ecology comes from the same root as our word "economics," the sub- 
ject that we now call ecology was not given a name until a century 
later, Man, being egocentric, began this type of study in his im- 
mediate surroundings. Not until long afterwards did he realize that 
man's economics is but a special case of the broader subject. In the 
words of Wells, Huxley, and Wells (1939), "Ecology is really an ex- 
tension of economics to the whole world of life." Economics and 
sociology might be thought of as the "ecology of man" in a broad sense. 
The realization that the relations of man to his environment, both 
physical and social, form a distinct and most important study is re- 
flected in the increasing use of the term human ecology in sociology 
and in other fields. 

Our goal in ecology is to understand the interrelations of organisms 
and their environments under natural conditions. Many biologists 
of all sorts have tended to lose this point of view. As Elton (1939) 
stated, "The discoveries of Darwin, himself a magnificent field 
naturalist, had the remarkable effect of sending the whole zoological 
world flocking indoors, where they remained hard at work for fifty 
years or more, and whence they are now beginning to put forth cau- 
tious heads again into the open air!" Laboratory tests and field ex- 
periments are, of course, used in studying the reactions of animals and 
plants as an aid to understanding their actual and possible behavior 
under natural conditions. Modern ecology thus goes beyond the mere 
description of the habitat, or the listing of its inhabitants, to an anal- 
ysis of causal relationships and a coordinated understanding of con- 
structive and destructive processes in the community. 

THE MEANING OF ENVIRONMENT 

In referring to the natural environment one tends to think first of 
the broad aspects of the landscape, such as water, soil, desert, or 
mountain. These types of environment can be more exactly described 
in terms of physical influences differences in moisture, temperature, 
texture of material, and the like and of biological influences. Other 
organisms form part of the environment just as much as the soil or the 
rocks. No animal can live entirely as a hermit; every animal must 
have other organisms within its range to serve as food. All animals are 
dependent directly or indirectly upon green plants. Many plants are 
dependent upon animals those that require pollination by insects, for 
example. Some green plants could live independently for a tinie, 
deriving their energy from the sun and their nutrients from the soil, 



4 Viewpoint of Modern Ecology 

but, as soon as seedlings start to grow, relations of competition appear. 
Every organism thus has other organisms as a necessary, or an un- 
avoidable, part of its environment. Animals and plants compete 
with each other, devour, or aid one another. Fellow inhabitants 
cannot be disregarded as part of the environment, as is clearly ap- 
parent in a thick stand of trees (Fig. 1.1) or in the slum conditions of 




U, S. Forest Service 

FIG. 1.1. Grove of giant redwoods near Crescent City, California, showing the 

intense competition of the trees with each other and their profound influence on 

the conditions beneath the forest canopy. 



The Critical Environment 5 

a sea-bird rookery (Fig. 1.2). In other situations the effects of the 
presence of other animals and plants are more subtle. Although such 
influences cannot be photographed, they are often just as crucial, and 
the environment must be understood to include both physical and 
biological influences whether conspicuousorobsc^iTe^ 






. *5-*." : -.- Jf . , >" 

' '' '"'"' 






FIG. 1.2. Gannet rookery on Bonaventure Island, Quebec, showing the keen 
competition for nest sites. 

The Critical Environment 

Are the necessary dealings of the organism with the environment 
usually successful? Can all living things find their needs in the out- 
side v/orld and avoid dangers? In most instances a large fraction of 
the young plants and animals produced are unable to cope with the 
environment, and relatively few survive. A striking example of the 
magnitude of natural mortality is found in the accompanying data 
from a study of the growth of mackerel eggs and young off the east 
coast of the United States (Sette, 1943). 

Stage of Young Mackerel Duration Mortality 

Spawning to 10-mm length 40 days 14% per day 

Transition to post-larva few days 30% per day 

Post-larva to 50-mm length 40 days 10% per day 



6 Viewpoint of Modern Ecology 

These values show that mortality was high throughout development 
and that a particularly critical period occurred during the transition 
to the post-larval stage. Out of each million mackerel eggs spawned 
in the area of this investigation only four survived on the average to 
reach a size at which the young fish could forage for themselves 
effectively. Others would, of course, succumb before they grew to 
spawning size. 

The U. S. Fish and Wildlife Service estimated that early in 1944 
there were roughly 125 million ducks in North America. These ducks 
ordinarily produce 10 to 16 eggs per pair. If all the adults and all the 
eggs produced in 1944 had survived, there would have been about 
900 million ducks inhabiting the continent in 1945. Actually the 
population was not significantly larger at the next census, early in 
1945, with again about 125 million ducks present. This means that 
something like 775 million ducks had died during the year. The 
sportsman's kill, which is estimated from licenses and bag reports, was 
about 20 million during 1944, or less than 3 per cent of the total 
mortality ( Griscom, 1947 ) . Again it is obvious that a very large por- 
tion of this bird population died during the year, and most died of 
"natural causes" not by the hand of man. 

One process developed during the course of evolution that acts to 
insure the survival of species in the face of this high mortality is the 
production of huge numbers of young. The prodigious fertility of 
nature is a measure of the destructive action that is received from the 
environment. In species in which each pair produces thousands or 
millions of eggs or seeds, only two survive on the average to reach 
the adult condition as long as the population is not continually in- 
creasing. Many examples could be given of the huge excess number 
of young that are produced and the small number that survive 
(Fig. 1.3). Insects are notoriously prolific. The queen termite in an 
African species after only one mating is reported to lay eggs at the 
rate of one every few seconds and to continue egg laying at this 
terrific pace for the rest of her life. A calculation shows that at this 
rate each termite queen would lay approximately 30,000 eggs per day 
or 100 million in her lifetime (Wheeler, 1923). According to esti- 
mates, an oyster may discharge 500 million ripe eggs in one spawning. 
If all these eggs developed into mature oysters and all subsequent 
progeny survived, after only four generations we would have a pile of 
oysters about eight times the size of the earthl 

Plants also have the capacity to reproduce at very high rates, and, 
if all spores or seeds grew to maturity, plant populations would in- 
crease with tremendous rapidity. Pine trees liberate such quantities 



The Critical Environment 7 

of pollen that a yellowish dust is often deposited on all surfaces in the 
vicinity. A tropical American orchid of the genus Maxillaria is re- 
ported to produce as many as 1,756,000 seeds per capsule. Spores are 
produced in astronomical numbers by many lower plants. A conserv- 
ative estimate made for the downy mildew, Sclerospora, which attacks 
maize in the Philippines, indicates that as many as 6 billion conidia 
(spores) are liberated from the fungi parasitizing a single maize 
plant during one night. Since discharge of conidia continues night 
after night for months, the intensity of the reproductive capacity of 
. these species surpasses the imagination (Weston, 1923). 






an/ Eschmeyer, 1938 

FIG. 1.3. Schematic representation of natural mortality in small-mouthed black 
bass, Only a few fish survive to maturity from hundreds of eggs laid. 

The adaptations of some plants and animals are more specifically per- 
fected to their environments, and their reproductive rates can con- 
sequently be relatively low, but even slow breeders tend to increase. 
If the elephant produces only six young in its life time, as has been 
estimated, these creatures would nevertheless eventually increase to 
such numbers, if all progeny survived, that they would cover the 
whole surface of Africa. An even slower breeding rate is exhibited 
by the California "big trees" (Sequoia gigantea] which do not pro- 
duce their first seeds until they are 175 or 200 years old! And yet 
even this species has the capacity to enlarge its population. 

Since no species of plant or animal goes on increasing indefinitely, 
we are forced to the conclusion that, whether reproduction rate is fast 
or slow, supernumerary individuals are killed off. Usually most of 
the young produced die at an early age. In the terms of the insurance 



8 Viewpoint of Modern Ecology 

salesman, their life expectancy is low. Man is a notable exception. 
Civilization has brought about an increasingly higher life expectancy 
in man and with economic and sociological consequences to which 
we must adjust. 

The great majority of plants and animals in nature die young, not 
because of flaws in their internal mechanism, but because of their 
failure to cope successfully with the external world. When fish eggs 
are raised under the best conditions in a hatchery, most of them sur- 
vive; very few die because of a mechanical or a physiological break- 
down in their embryological development. When seeds are sown 
in a well-prepared greenhouse bed, germination of most of the popu- 
lation is expected. On a bag of grass seed, you will usually find a 
statement that tests of the seed have shown a germination of 80 to 95 
per cent. In a laboratory culture the larvae of a marine copepod were 
raised to the adult stage with an 80 per cent survival, although 
a survival of less than 0.5 per cent would have been sufficient to main- 
tain the population on the basis of the number of eggs laid (Johnson 
and Olson, 1948 ) . In the laboratory the copepod was protected from 
predators, diseases, and the exigencies of the physical and chemical 
factors of its usual environment, which would normally have killed 
off more than 995 out of every 1000 young animals. 

Of course, some individuals are born with congenital flaws in their 
anatomy or physiology. Little information exists on the frequency 
with which lethal genes occur in wild populations, but their influence 
is probably very slight. Studies of wild fruit flies indicate that only 
about 20 per cent of the eggs laid fail to develop into adult insects 
because of genetic causes. Most of these "lethals" succumb because 
the genetic changes cause failures in their relations to the environment, 
rather than failures in their internal adjustments. 

The few animals or plants that have escaped death long enough 
to reach maturity are leading a very precarious existence. A sword 
of Damocles hangs over their heads! The threat is ever present that 
the action of the environment may become a little more severe and 
wipe the population out. This sometimes happens. A striking ex- 
ample of wholesale destruction of a bird population occurred in Min- 
nesota when hundreds of Lapland longspurs were killed during a late 
winter storm (Fig. 1.4). Another illustration of mass mortality is the 
suffocation of fish under the ice of a lake (Fig. 1.5). In these in- 
stances certain physical features of the environment became too 
severe and the entire local population was killed. 

Sometimes a biological influence goes beyond tolerable limits. 
Older residents of northeastern , United States tell of Sunday excur- 



The Critical Environment 




^JOT^* ; ^ -V:;!. *> 

^m^^'^l:^^^^^^;^ :\^ii ,,,^ 






. 

FIG, 1,4. Dead Lapland longspurs on the frozen surface of a Minnesota lake 
after a late winter storm. 




Prescott, 1939, AAAS Pub. No, 10 

FIG. 1,5 Masses of dead fish washed ashore after the breakup of the ice in Lake 
East Okoboji in northwestern Iowa. Bass, perch, carp, and other fish were killed 
by the depletion of oxygen under the ice following the decay of a heavy growth 

of algae. 



10 Viewpoint of Modern Ecology 

sions into extensive groves of edible chestnut trees to gather nuts. 
Now an edible chestnut tree is a rarity because a blight on this species, 
caused by a fungus, spread throughout the northwestern states and 
wiped out the chestnut groves between 1910 and 1930. 

If the slight, but crucial, increase in the destructive action of the 
environment extends beyond a local area to the whole range of a 
species, its effect becomes much more serious, and it may even result 
in the extinction of the species. This, too, has happened, not once, 
but many times. Within the memory of man such species as the 
Labrador duck and the passenger pigeon have become extinct; many 
more, such as the American bison, would have been entirely wiped 
out if it were not for protected preserves. No one knows how many 
species during the ages failed to meet the challenge of the environ- 
ment. We do know that about 21,000 species of extinct vertebrates 
and an even larger number of extinct higher plants have been de- 
scribed. If we add to this figure a guess as to the number of extinct 
vertebrates and higher plants whose remains were never found, and 
of the extinct species of invertebrates and of lower plants, we shall 
have some impression of the precariousness of existence in this world. 
As if the destructive action of the environment were not serious 
enough under natural conditions, man has added immeasurably to it 
as civilization has "advanced." As will appear many times in our 
further discussions, man need not necessarily be the destroyer. With 
an adequate understanding of ecological principles he can utilize 
many natural resources without impairing the*n. In some instances 
the abundance and variety of the natural fauna and flora have im- 
proved as a result of man's activities. If we include cultivation, a 
very great development of plant and animal life has, of course, been 
brought about by the hand of man. Unfortunately the intelligent use 
of biological resources has not been the rule. Whole populations of 
animals and birds have been slaughtered for their fur or their feathers. 
Grasslands have been ruined, forests have been cut down (Fig. 1.6). 
Besides direct devastation of much of our natural vegetation and 
wildlife, we have wrought even more harm through causing serious 
pollution of lakes, rivers, and harbors, irreparable damage to the soil, 
and loss of ground-water reserves (Fig. 1.7). The sad story of man's 
destruction of his own natural resources throughout the world has 
been effectively told. The need for intelligent conservation and the 
steps being taken in the United States to achieve it have been sum- 
marized by Gustafson, Guise, Hamilton, and Ries (1949), by G. H. 
Smith (1950), and others. Suffice it here to stress once more the fact 
that animals and plants in nature are living dangerously. The natural 



The Critical Environment 







U. S. Forest Service 

FIG. 1.6. Advanced degree of land destruction near Leadville, Colorado, where 

forest has been ruined by clear-cutting, and the complete removal of trees has 

resulted in gullying and serious soil erosion. 




FIG. 1.7. Soil erosion and consequent farm abandonment in Oklahoma, showing 

disastrous economic and sociological effects of agricultural practices carried on 

without regard to the ecological limitations of the region. 



12 Viewpoint of Modern Ecology 

environment causes the death of most young organisms brought into 
the world, and sometimes it may vary so as to kill all members of a 
population. Man is an unavoidable part of the environment. With 
intelligence he may help guard the danger spots he may even improve 
the productivity of natural environments. Without intelligence man 
accelerates destruction. 

In the preceding examples of mass mortality and extinction the 
devastating action which the environment can have, with and without 
the presence of man, is obvious to all. For every spectacular instance 
of this sort, there are thousands of instances in which no cataclysmic 
destruction is taking place, but in which the forces of the environ- 
ment are nevertheless exerting a crucial influence in some subtle or 
obscure manner. 

This undercover work of the agents of the environment may perhaps 
be most easily appreciated first in its action on individual species. 
The geographical range of each species, for example, is controlled by 
the pruning action of external forces. Every species is pressing 
against its boundaries and is always tending to extend its range. 
It is held in check by the physical and biological factors of the environ- 
ment which kill off those individuals which spread out too far into 
areas where conditions arc no longer tolerable. During periods when 
no unusual fluctuations occur at the limits of the range inhabited by 
a species, the destructive effect of the environment may not be spec- 
tacular because ordinarily only a few individuals are eliminated at any 
one time or place. 

The environment may thus control crucially, but quietly, the 
geographical range of a species. Fluctuations in the environment 
may even allow the limits of distribution to change somewhat as 
conditions vary without a particularly noticeable mortality. One 
example of that possibility is found in the commercial fishery records 
for the landings of the weakfish along our middle Atlantic coast. 
This fish, which is sold in New York markets as "sea trout," was 
caught only in waters south of Cape Cod previous to 1895. During 
that year and in succeeding seasons weakfish were taken north of the 
Cape, and by 1901 the catch was large enough to support a regular 
fishery. By 1907, however, the numbers of this fish had dropped 
again to such a low figure that the fishery was abandoned, and never 
established again north of Cape Cod. There was no dramatic 
change in the ocean waters around the Cape during those six years. 
Some subtle, unnoticed variation in the ecological conditions allowed 
the weakfish to extend its range a short distance for a little while, and 
then caused the northern limit of the species to be drawn back again. 



Ecology of Plants and of Animals 13 

However, the environment sometimes changes so as to allow huge 
variations in the numbers of organisms, such as plagues of mice or 
insect pests, as will be discussed later. More generally, however, 
the fluctuations in the surroundings and in the abundance of plants 
and animals are less violent. In the great majority of these unspec- 
tacular oscillations it is nevertheless the environment that is exerting 
the vital control of the numbers of each species. In order to discover 
the causes of fluctuations, which are sometimes of great economic 
importance, a clear understanding of the essential relations between 
the organism and its environment must be obtained. 

The everyday action of the environment in curtailing the geographi- 
cal spread and the abundance of organisms has other significant 
effects. The production of more young plant seedlings and young 
animals than can survive causes a continual struggle for existence, 
with the resulting survival of the fittest. These are essential factors 
in the evolutionary process as was pointed out by a prominent "ecol- 
ogist" by the name of Darwin, long before the term ecology came into 
common use. Through its influence on the individual the environ- 
ment indirectly controls the natural community. It determines the 
complexion of the whole population by determining which species 
can exist in each area and the relative numbers of each. Through the 
control of feeding, growth, and other activities of each living com- 
ponent, the environment regulates the dynamic operation of the 
community. 

THE DEVELOPMENT OF ECOLOGY 

The Ecology of Plants and of Animals 

The term "oecology" was first used by the German zoologist Haeckel 
in 1869. The word did not appear again until 1895, when a report 
on ecological plant geography was published by Warming, a Danish 
botanist. The term in its modern spelling was taken up again later 
by the zoologists. In those early days and for a long period there- 
after botanists and zoologists were often working quite separately. 
It is not surprising, therefore, that plant ecology and animal ecology 
tended to develop independently, but it is unfortunate that this divi- 
sion into two separate fields tended to persist. We have now come to 
realize that a proper understanding of the ecology of animals neces- 
sarily involves a consideration of the plants of the environment, and 
that a study of the ecology of plants would be incomplete without in- 
cluding the influence of animals. However, ecologists, and biologists 



14 Viewpoint of Modern Ecology 

in general, have been slow to point out that many of the principles of 
ecology are the same for both plants and animals. 

The modern ecologist strives to understand the fundamental in- 
fluence of the factors of the environment and to delineate such gen- 
eral concepts as limiting action, competition, population growth, and 
the like. These principles may be applied to the plants or to the 
animals of the region under investigation, but for many concepts, 
such as those of the food chain and the dynamics of energy exchange, 
both the plants and the animals must be considered. The term ecol- 
ogy thus necessarily includes the interrelations of all kinds of organisms 
with the environment. 

The Ecology of Habitats and of Individuals 

In the early development of the subject, one group of ecologists 
concentrated on the relations of the habitat. They strove toward a 
better description of the habitat and of the influences of the habitat 
on the plants and animals that lived there. First the physical fea- 
tures and then the biological influences were investigated. The study 
of the habitat and its effects is often spoken of as habitat ecology. 

The work of other early ecologists, instead of beginning with a 
description of environments, took its departure from an investigation 
of the individual plant or animal. Attention was focused on the needs 
and the reactions of the organism and the influence of environmental 
factors upon it. Thus was developed a study of the ecology of the 
individual, or to use a specially coined term, autecology. 

The Ecology of Populations and of Communities 

While the habitat ecologists were hard at work, other biologists with 
ecological leanings were turning their attention to the fact that new 
interrelations appear when groups arise. No animal or plant lives as 
a completely isolated individual. When groups of the same species 
are formed new effects appear. A simple example will illustrate this 
point. Suppose that 100 trees are growing as individuals widely 
spaced in a pasture. The shade from each of these trees will move 
around during the course of the day so that the ground beneath the 
trees will receive direct sunlight at least for a time each day. On the 
other hand, if these same 100 trees were growing close together in a 
grove, the shadow of one would overlap that of the next, with the 
result that continuous shade would exist underneath. The effect of 
the trees in the grove on temperature of the soil, evaporation, and 



Ecology of Populations and Communities 15 

wind conditions would be entirely different from that produced by 
widely spaced trees. The interdependencies resulting from an aggre- 
gation of individuals of the same species may become very complex, 
as for example in an insect colony. The size of a population of ani- 
mals or plants and its rate of growth are regulated by the reactions 
of the members of the population to each other and to the environ- 
ment. The study of these and similar relationships of groups of or- 
ganisms is termed population ecology. 

When several species of plants and animals are present, as is usual 
in a natural community, still further complications arise. In the 
example cited above a very different vegetation would exist beneath 
the 100 trees growing as isolated individuals from that found on the 
floor of a dense grove. The species of animals associated with the 
plants will also differ widely in the two situations. Widely varying 
combinations of plants and animals coexist in the many different 
habitats of the world. It is found that certain species live together 
in mutual adjustment, and these are spoken of as a natural community. 
The study of the relationships of the animals and plants making up a 
community is termed community ecology or, again to use a coined 
term, synecology. 

The development of the different viewpoints in ecology mentioned 
above is due in part to the fact that the plant ecologist and the animal 
ecologist have tended to work rather independently, The community 
of plants is perhaps more obvious than the community of animals. 
With plants the vegetation as a whole is often more striking, whereas 
with animals the individuals tend to be considered first. Since animal 
communities were generally less apparent, they were delineated at a 
later date. The result of these influences was that animal and plant 
communities were first thought of and studied quite separately. 

As ecology developed, it came to be realized that the animals of an 
area do not constitute a community entirely distinct from the plants 
of that area. It is true that in some situations, as for example in the 
desert, the interrelations between the animals and the plants may be 
less critical than the dependence of both upon the physical factors of 
the environment. Nevertheless, because of the fundamental depend- 
ence of animals upon green plants and the influences commonly 
effective in the reverse direction, the plants and animals of a region 
should be considered as one integrated community. The animal 
taxonomist lists the fauna in a region and the plant taxonomist records 
the flora. The fauna and flora together are spoken of as the biota of 
the region. In an analogous fashion the modern ecologist considers 
the integrated community of plants and animals as the biotic com- 



16 Viewpoint of Modern Ecology 

munity. It cannot be emphasized too strongly that the community 
exists because of the suitable reactions of the individuals which make 
up the community. Therefore, no sharp line exists between the ecol- 
ogy of the individual and community ecology. All the foregoing 
concepts will be discussed more fully and illustrated in subsequent 
chapters. 

The Ecological Complex 

We have traced the development of the concept of the biotic com- 
munity as the complete assemblage of interdependent plants and ani- 
mals inhabiting an area. As a further step it came to be realized that 
the physical conditions of the area must be considered for the com- 
munity just as for the individual. (fThe organisms interact with each 
other and also with the physical conditions that are present. Thus 
organisms and the physical features of the habitat form an ecological 
complex, or, more briefly, an ecosystem. J 

In ecology, as in other subjects, the descriptive view appeared first. 
Lists of the animals and plants present in characteristic situations 
were prepared, and values were reported for the physical and chemical 
conditions of the areas. In time all the important habitats became 
subjects of special study, including the forests, the grasslands, the 
deserts, the mountains, and the aquatic regions. Investigations of 
certain definite habitats gave rise to such sciences as forestry, ocean- 
ography, and limnology. In other instances studies of special types 
of environment were not given specific names. 

As various characteristic environments and their biotas were in- 
vestigated more intensively, the realization developed that the eco- 
logical complex should not be viewed as a static group of animals and 
plants with the accompanying climatic conditions. The ecosystem is 
not a museum group remaining immovable and unchanged as genera- 
tions of observers pass by the plate glass windows. The community 
cannot continue to exist without exchanges and interdependencies 
any more than the individual plant or animal can. The community, 
as well as the individual organism, is "something happening." Thus 
the functional viewpoint gained momentum as modern ecology de- 
veloped. Sears (1939) nearly emphasized this point by stating, 
"When the ecologist enters a forest or a meadow, he sees not merely 
what is there, but what is happening there." 

As the action of the environment was studied in further detail it was 
found convenient to list its influences as ecological factors. The stu- 



The Ecological Complex 17 

dent may have at first gained an impression of all the factors of the 
environment impinging upon the organism in a one-sided action. 
With further understanding of the natural situation, however, it came 
to be realized that not only do these factors affect the organism, but 
also the organism affects its environment. This reciprocal action is 
seen first of all in relation to the physical features of the habitat. For 
example, light plays a part in controlling the growth of trees, and 
at the same time the trees control the amount of light beneath 
them. Similarly, the dissolved nutrients and oxygen in pond water 
aflect the growth of the aquatic organisms that live there, but the very 
activity of these organisms in turn modify these factors. The growth 
of plants depletes the supply of nutrient salts. The respiration of ani- 
mals consumes oxygen and increases the amount of carbon dioxide in 
the water. Thus the environment receives materials from the organ- 
isms living in it and loses material to them as they grow. The fact 
that regular changes in the environment are brought about by the life 
activities of the inhabitants was especially emphasized in a series of 
Lowell Institute lectures given by A. C. Redfield in 1941, which he 
entitled The Physiology of the Environment. 

The animals and plants modify the biological features of their 
environment just as they do the non-living factors. ; In thinking of the 
activity of a carnivore, such as a fox preying on rabbits, we realize first 
perhaps the influence of the predatory action in killing the rabbits and 
depleting their numbers^ But by turning the picture around it be- 
comes clear that the abundance of rabbits also influences the fox popu- 
lation. A rapid growth of the individual foxes and a high rate of 
reproduction is made possible if the supply of rabbits for food is large, 
but the number of foxes may be drastically curtailed if rabbits be- 
come extremely scarce. In the same way when we observe a flock of 
sheep in a pasture, we tend to think first of the activity of the sheep 
in grazing down the grass. It is also true, however, that the sheep's 
sharp teeth are clipping off tree seedlings which may be sprout- 
ing in the turf and the animals are adding manure to the pasture. 
If it were not for the presence of the sheep, the continued existence 
of the turf would often no longer be possible. In many situations 
trees would seed in, and as a forest grew up the turf would be killed 
off. Clearly the sheep and the plants of the turf form an integrated 
system. 

The concept that organisms and their environment form a recipro- 
cating system represents the viewpoint of most modern ecologists. 
In every natural situation the environment affects the organisms pres- 



18 Viewpoint of Modern Ecology 

ent and to a greater or lesser extent, the organisms affect the environ- 
ment. The accompanying terms were proposed by Clements to de- 
scribe the several aspects of the foregoing relationships. 

"Action" = habitat > organism 
"Reaction" = organism > physical factors 
"Co-action" = organism organism 

The community, which Sears (1950) aptly refers to as "the living 
landscape," maintains itself as a working unit with all the necessary 
exchanges going on, more or less in balance, but in a dynamic and not 
a static balance. The functional concept of the community and of 
the two-way reaction between the environment and its inhabitants 
carries us far beyond the descriptive view. The improvement gained 
from this modern approach in ecology is analogous to the better 
understanding of the conditions inside an individual animal or plant 
that is obtained when the physiologist's viewpoint and technique are 
added to those of the anatomist. Modern ecology might thus be 
thought of as the "physiology" of the ecological complex in the sense 
that it deals with the functional aspects of the interactions, exchanges, 
and adjustments of the members of the community arid of their 
environment. 

THE SCOPE OF ECOLOGY 

Taylor (1936) has said "Ecology is the science of all the relations 
of all the organisms to all their environment." Since the plant and 
animal inhabitants may be very abundant and diverse, and since en- 
vironmental conditions are extremely variable, the possible scope of 
ecology becomes very great. The central task of ecology, however, 
is to delineate the general principles under which the natural com- 
munity and its component parts operate. These may then be applied 
to the interpretation of the activities of the particular plants and ani- 
mals present under the existing specific conditions of a given stiuation. 

Although the fauna and flora of an area must be identified and 
enumerated, and although the physical forces at work in the area must 
be recognized, neither an account of the biota, nor a description of 
the habitat constitutes an ecological investigation. Similarly, if a man 
arises at daybreak and makes a list of the birds he sees without any 
consideration of the relation of the occurrence of these species to 
other factors, he is not an ecologist. Modern ecology is concerned 
with the functional interdependencies between livinfe things and their 



The Scope of Ecology 19 

surroundings. Ecology is primarily a field subject. Nevertheless, a 
knowledge of the principles and problems of ecology should be ac- 
quired before attempting to evaluate a natural situation, where the 
multiplicity of ecological activities may be bewildering. Many eco- 
logical relationships can be effectively analyzed under the simplified 
and controlled conditions of the laboratory. 

Because of a lack of understanding of ecological principles the 
efforts of well-intentioned conservationists and agriculturalists are 
frequently badly misdirected. A story is told of certain sheep ranchers 
who became convinced that coyotes were robbing them of their young 
sheep. As a result, the community rose up and by every possible 
means slaughtered all the coyotes that could be located for miles 
around. Following the destruction of the coyotes, the rabbits, field 
mice, and other small rodents of the region increased tremendously 
and made serious inroads upon the grass of the pastures. When this 
development was realized, the sheep men executed an about-face, 
abruptly stopped killing the coyotes, and instituted an elaborate pro- 
gram for the poisoning of the rodents. The coyotes filtered in from 
surrounding areas and multiplied, but finding their natural rodent 
food now scarce they were forced to turn to the young sheep as their 
only available source of food! 

An understanding of ecological principles provides a background 
for further investigations not only into the fundamental relationships 
of the natural community but also into sciences dealing with particular 
environments such as the forest, soil, ocean, or inland waters. Many 
practical applications of ecology are found in agriculture, biological 
surveys, game management, pest control, forestry, and fishery biology. 
Knowledge of ecology is critically important for intelligent conserva- 
tion whether in relation to soil, forest, wildlife, water supply, or 
fishery resources. 

Ecology is significant also in a wider sense for us as citizens. It 
gives us an insight into how the world works. In addition, man him- 
self is a most important element in the environment. Man almost 
always has a modifying influence, and, without proper regulation, he 
often has a destructive effect. Man is himself an organism with an 
environment, and this fact has been particularly emphasized in the 
development of human ecology. A knowledge of the general prin- 
ciples of ecology thus provides a background for the understanding 
of human relations just as a study of general zoology is necessary as a 
groundwork for medicine. Like other animals man is influenced by 
the physical features of his environment, he is absolutely dependent 
upon other species, and he must adjust to other individuals of his 



20 Viewpoint of Modern Ecology 

own species, At the moment man is suffering from lack of these 
adjustments. 

APPROACH TO THE STUDY OF ECOLOGY 

The study of ecology is best begun through the analytical approach. 
This involves the delineation of the individual influences of the en- 
vironment and the recognition separately of the various activities of 
the organisms present as steps toward building an understanding of 
the entire dynamic interaction between the complete environment 
and its inhabitants. 

The fundamental relationships are most readily grasped by analyz- 
ing the simplest situations first. Contrast, if you will, the ecological 
dependencies of an alga living near the surface in the open ocean 
with those of a tree growing on land. The tissues of the alga receive 
their energy supply directly from the sun and they carry on their inter- 
change of materials directly with the surrounding water, which is 
uniform and extremely constant in respect to the ecological factors 
concerned. The tree, on the other hand, is partly in the light and 
partly in the dark. Part of the tree is surrounded by the atmosphere 
with its widely fluctuating temperature and humidity; part is in the 
soil, where it is subject to a very different temperature and is alter- 
nately flooded with air and with water. The part of the tree that is 
above ground must deal with one set of organisms, and the part of the 
tree below ground is concerned with an almost entirely different set. 

Another reason for adopting the analytical approach is that this 
procedure is more likely to reveal limiting factors. All animals and 
plants tend to grow, to reproduce, and to disperse until checked by 
some influence of the environment. The factor that first stops the 
growth or spread of the organism is called the limiting factor. It is 
not always easy to single out the limiting factor, and sometimes two 
or more factors combine to provide the limiting influence. Never- 
theless, it is extremely desirable whenever possible to determine what 
agent or agents control the natural tendency of the plants and animals 
present to increase in size, numbers, and range. In the investigation 
of any natural area correlations will be found between features of 
the environment and the activities of organisms present. Analysis of 
the action of individual influences at work in the habitat is necessary 
in determining which of the correlated factors are actually causal 
factors. Suppose, for example, that we discovered a correlation be- 
tween the occurrence of the factor A and organism B. Should we 
conclude that A causes B? 



Approach to the Study of Ecology 21 

A-+B 

It might very well be that no direct causal relation exists between 
A and B whatsoever, but that both are controlled by a third influ- 
ence, C. 

A B 

v 

Or factor A may influence C, which in turn influences B, thus: 
A B Temperature Plant 

\ / \ . / 

C Moisture 

In considering the factors of the environment separately in order to 
distinguish and to measure the influence of each, we must remain 
thoroughly aware that in nature the factors are never acting alone. 
Animals and plants are subject to many influences at the same time, 
and the effect of one factor is often modified by action of other factors. 
The "real life" of the organism, on which its growth, distribution, and 
multiplication depend, necessarily involves the simultaneous and con- 
tinuing impact of all existing factors and also influences that occurred 
at earlier stages in the organism's experience. 

An important difference exists in the extent to which factors can be 
modified by living organisms. Some features of the environment are 
largely unaffected by the activities of the organisms present; these are 
unmodifiable or conservative factors. The salinity of the ocean is an 
example of a conservative factor. The volume of the ocean is so great 
that, although animals and plants living in it are continually adding 
or withdrawing salts, the amounts have an immeasurably small effect 
upon the total salt content of the water. The modifiable or non-con- 
servative factors of the environment are susceptible to change caused 
by the inhabitants of the area. The oxygen in a small pond, for ex- 
ample, may be so depleted by the respiration of a large population of 
fish that an unfavorable or even a lethal condition for the fish is pro- 
duced; or the concentration of oxygen in the pond may be increased 
by the photosynthesis of algae a modification that will benefit the 
fish. Heather (Calluna) tends to increase greatly the acidity of the 
soil in which it is growing, and this condition favors the further de- 
velopment of this plant, but it is unfavorable to most other plants. 
Through modification of its own environment, heather often comes to 
dominate the vegetation in large areas, as may be seen in Jutland. 

No sharp division exists between modifiable and unmodifiable fac- 
tors. All gradations exist, and a given factor may be modifiable in 



22 Viewpoint of Modern Ecology 

some situations or for some organisms and quite unmodifiable for 
others. In the succeeding chapters the more general, often unmodi- 
fiable, factors will be scrutinized first, and will be followed by a 
discussion of the more commonly modifiable factors. This will lay 
the foundation for a consideration in the later chapters of the com- 
position and functioning of the community as a whole. 



2 

The Medium 



The first of the physical features of the environment to be considered 
will be the medium that is, the material which immediately ;. sjucrounds 
the organism and with which it has its all important exchange. At 
first sight one might think that many diverse media exist. Some 
organisms live in the soil, and some in ponds; some thrive in manure 
piles, and others enjoy a successful existence in the blood stream of 
vertebrate animals. Certain nematodes live in vinegar, and a fly 
larva of the genus Psilopa grows in petroleum. Once during a de- 
partmental gathering at Cambridge University a member of the staff 
entered the room waving a journal in which the habits of this larva 
were reported. "Look here," he said, "in this report an insect is 
described which lives in petroleum. The first thing you know it will 
parasitize our motor cars!" 

The medium in each of the above examples, and indeed that for 
organisms in every natural situation, is either a liquid or a gas, and it 
i.s usually air or water. Although animals and plants inhabiting soil 
or mud may at first appear to be exceptions, a closer scrutiny shows 
that a film of air or water around each organism is actually the ma- 
terial in immediate contact with it. An enlarged view of the small 
animals living in the wet sand of the seashore shows that their essen- 
tial exchange is with the water percolating between the sand grains 
and that the medium for these animals is sea water, not sand (Fig. 
2.1). The term medium is thus used in a strict sense and is distin- 
guished from the substratum, or surface on or in which the organism 
lives. 

The existence of air and water as the fundamental media divides 
the world into two major environments: terrestrial and aquatic. The 
media are not completely isolated from each other, however; some of 
the atmospheric gases are dissolved in all natural waters, and some 
moisture is present almost everywhere in the atmosphere. Differences 

23 



24 



The Medium 



in the amount of intermixture play a part in subdividing the terrestrial 
environment into arid and humid climates and the aquatic environ- 
ment into stagnated and aerated water. Transition areas of special 
interest exist such as swamps and the tidal zonewhere sometimes 
one medium and sometimes the other dominates the scene. Dams 
built by beavers occasionally result in the flooding of large tracts of 



1mm. 




Pennak, 1939, AAAS Pub. No. 10 

FIG. 2.1. Enlarged diagram of sand in a beach habitat showing water-filled spaces 

between the grains. 1 = rotifers, 2 = gastrotrichs, 3 = tartigrade, 4 nematode, 

5 = harpacticoid copepods. 

land, transforming them from terrestrial to aquatic habitats (Fig. 
2.2); conversely, the growth of vegetation often tends to fill up a 
shallow pond, gradually converting it into a swamp, and eventually 
into dry land. Ordinarily, however, the medium is a highly inde- 
pendent factor, for rarely do the activities of organisms cause a change 
from one basic medium to the other. 



CONTRASTING QUALITIES OF AIR AND WATER 

The two fundamental media are very different in nature, and this 
difference has important ecological consequences. ^Air is composed 



Contrasting Qualities of Air and Water 25 

of 79 per cent nitrogen, 21 per cent oxygen, 0.03 per cent carbon 
dioxide, and several other gases in much smaller quantities^ These 
gases are not chemically combined, but exist as a simple physical 
mixture. Water, by contrast, consists primarily of a single compound, 
H 2 O. There is nothing especially unusual about the physical and 
chemical properties of air and the gases of which it is composed. 




U. S. Forest Service 

FIG. 2.2. Pond formed by beaver dam (left foreground), showing beaver house 
(right center) and trees felled and stripped by the beavers. Cochetopa National 

Forest, Colorado. 

Water, on the other hand, is a unique substance from the ecological 
viewpoint. H. B. Bigelow, formerly director of the Woods Hole 
Occanographic Institution, when lecturing on oceanic biology once 
stated: "The most important fact about the ocean is that it is full of 
water!" 

The unusual qualities of water are discussed in detail by Henderson 
(1924) in his classic book, The Fitness of the Environment. Suffice 
it here to mention a few of the attributes of water that have a special 
ecological importance. In the first place water is the most abundant 
substance of the earth's surface, covering more than 70 per cent of the 
area of the globe. Because the oceans are about 2% times more 
extensive than the land, and because they are habitable throughout 
their depth, the sea provides more than 300 times the living space. 
Water has a higher specific heat, latent heat of fusion, and latent 



26 The Medium 

heat of evaporation than any other common substance. These facts 
play a very important role in the heat regulation of organisms them- 
selves and in the resistance of natural environments to temperature 
change. 

Another characteristic of water having crucial ecological significance 
is its relatively high freezing point. Because of the large amount of 
heat which must be given up before water can turn to ice and because 
of restricted stirring, oceans and lakes freeze only at the surface. 
Even ponds rarely freeze to the bottom. The temperature of the 
medium, therefore, can drop only to 0C in fresh-water environments, 
or to a few degrees lower in the ocean. The biological reactions of 
a great many plants and animals can still go on perfectly well at 
temperatures down to the freezing point of water. 

Another unusual quality of water is its power as a solvent; no other 
common substance compares with water in this respect. Many kinds 
of material can pass into, through, and out of the body of an organism 
in aqueous solution. Water provides a transporting medium that is 
versatile as a solvent but not too active chemically. Very consid- 
erable solution would take place if sulphuric acid ran in our rivers or 
coursed through our veins, but such a solvent would profoundly 
alter the materials that it carried. Furthermore, the extent of ioniza- 
tion of solutes in water is extremely high, providing the possibility 
of a great variety of radicals and of chemical recombination. Water 
has the highest surface tension of any common substance except 
mercury. This high surface tension has many ecological influences, 
involving the movement of water into and through organisms as well 
as the rise of ground water in the soil. 

Many of the foregoing differences and special qualities of air and 
water will be referred to again in connection with other factors of 
the environment. Confining ourselves for the moment to the simple 
physical differences between the two fundamental media, let us ex- 
plore the ecological effect of the difference in density of air and 
water. The densities of representative natural waters and of air, 
and the approximate average density of protoplasm are as tabulated. 

Pure water 1 . 000 g/cc at 4C 

Pond water 1.001 

Sea water (at salinity of 35%c) 1 . 028 

Air (at sea level) . 0013 

Protoplasm 1 . 028 

The density of protoplasm is closely similar to that of sea water and 
only slightly greater than that of fresh water, but it is more than 850 
times greater than that of air. Associated with this difference in 



Pressure 



27 



density are important differences in the pressure, inertia, viscosity, 
and mobility of the media. 



PRESSURE 

The difference in the densities of the media results in a great 
difference in the rate of change of pressure at increasing altitudes in 
the atmosphere and at increasing depths in the water. Near the 
earth's surface a rise of 300 m (1000 ft) in altitude results in a reduc- 
tion of pressure of about 25 mm Hg, or a relatively slight change in 

f- 25,400 m - Rocket-powered plane 
22 mm Hg I- 22,000 m - Stratosphere balloon 



235 mm Hg r 8840 m (29,000 ft) - Mt Everest 



AIR 



Rates of change: 
25 mm Hg/300 m 



310 mm Hg 

367 mm Hg 
413 mm Hg 



760 mm Hg - 1 Atmos. 



- 7000 m Vultures and eagles 

- 5800 m Wild sheep and ibex 

- 4860 m Highest human settlement, Tibet 

- 4420 m - Mt Whitney, California 



1920 m - Mt Washington, N. H. 
Sea level 



1 Atmos/10 m 



WATER 



370 Atmos. 



625 Atmos. 



925 m Deepest dive of bathysphere (Beebe) 
1400 m Deepest dive of benthoscope (Barton) 



3700 m Average depth of oceans 

4050 m Deepest dive of bathyscaphe (Houot) 



6250 m Ten species of animals taken by 
"Challenger" 



_ 10,500 m Various invertebrates taken by "Galathea" 
1086 Atmos. L 10,860 m (35,640 ft) Greatest ocean depth 

(Mariana Trench) 

FIG. 2.3. Range of pressure in air and water in relation to the distribution of life. 



28 The Medium 

pressure. In contrast, for every increase of 10 m (33 ft) in depth in 
the water, pressure is increased by 760 mm Hg, or 1 atmosphere. The 
tremendous pressures existing at the average depth of the ocean and 
in the ocean deeps are indicated in Fig. 2.3. At a depth of only 900 
m in the ocean Beebe's bathysphere was subjected to a total pressure 
on its whole surface of more than 7000 tons. It is not surprising that 
in his explorations of the earth man has not been able to descend into 
the ocean to depths much greater than 4 km, whereas he has ascended 
in the atmosphere to heights of more than 22 km. What are the 
ecological effects of these differences in pressure and in the rates of 
pressure change in the two media? 



Pressure Reduction with Altitude 

The reduction of pressure with altitude seems to be of little impor- 
tance for plants, invertebrate animals, and the lower vertebrates. 
Insects have been subjected experimentally to a reduction of pressure 
from 760 mm Hg to 0.0001 mm Hg without harmful effect. Similarly, 
frogs have withstood reduction of pressure down to 100 mm Hg. 
Beetles reach the highest meadows in the Himalayas and earthworms 
are found up to the snow line in the Andes. In most situations it is 
not the reduced pressure which limits the altitude at which plants and 
cold-blooded animals can exist. The distribution of these organisms 
up the sides of mountains is ordinarily stopped by other adverse fac- 
tors, such as low temperature, unsuitable soil, or lack of food, long 
before the influence of the reduced pressure is felt. 

For warm-blooded vertebrates the reduction of pressure with alti- 
tude becomes important primarily because of the lesser amount of 
oxygen present. It is true that at very high altitudes the thinness 
of the air renders flying more difficult, but the chief limitation imposed 
on birds, and also on mammals, is the impairment of respiration. 
Distinctly harmful effects are observed for man when the pressure of 
the atmosphere has been reduced to about half that normal at sea 
level. The highest permanent human settlement occurs in Tibet at 
an altitude of about 5000 m. Even the best adapted of other mam- 
mals are not found living permanently much higher than this altitude 
(Hesse, Allee, and Schmidt, 1951, Ch. 24). Although vultures and 
eagles have been reported at about a thousand meters higher, they 
probably remain at such altitudes for only short periods of time. 
Insects, which often abound near the rocky or snow-covered peaks of 
mountains, are undoubtedly blown there by the wind, a^ they could 



Pressure Increase with Depth 29 

not survive long under the conditions of the low temperature and 
lack of food, 



Pressure Increase with Depth 

If the relatively slight reduction in pressure with altitude in air is 
important to some organisms, the tremendously greater increase in 
pressure with depth in water might be expected to have serious con- 
sequences to all aquatic organisms, When early calculations were 
made of the magnitude of the pressure at the bottom of deep lakes, 
and particularly of the ocean, it was believed that the stupendous 
pressures would annihilate all living beings, so that the greatest depths 
in the aquatic environment must be lifeless, This conclusion seemed 
at first to be confirmed by the early explorations, but, with the im- 
provement of gear for investigating the bottom of the deep sea, animal 
life was gradually discovered at greater and greater depths in all the 
ocean basins (Fig, 3.5), In 1951 the Danish Galathea Expedition 
trawled 17 sea anemones, 61 sea cucumbers, 2 bivalves, and 1 
crustacean from a depth of about 10,500 m off the Philippine Islands. 
At this depth the pressure is 1050 atmospheres, or about 1 ton on each 
square centimeter, but this terrific weight of water does not crush the 
organisms living at that depth because the pressure is the same inside 
their bodies as outside. 

When most deep-sea animals are brought to the surface, they are 
dead or dying. The popular opinion is that they have been killed 
by a violent release of pressure. In reporting the work of the re- 
search vessel Atlantis, a newspaper once stated: "The sudden change 
of pressure when deep-sea fish are brought to a higher level in the 
ocean causes them to explode. The fragments are then put together 
again!" Fish with air cavities within their bodies do indeed expand 
when they are brought to a higher level, but most fish inhabiting the 
ocean abyss have no air bladders. Their death is due primarily to 
injury from the nets and to the change in temperature experienced in 
being brought to the surface. 

The effect of the pressure changes in the aquatic environment is 
very different for organisms with and without air cavities. Aquatic 
plants that live at levels considerably below the surface stratum and 
the great majority of deep-sea animals do not have gas-filled spaces 
in their bodies. Since any cavities in these organisms are completely 
filled with fluids, no mechanical deformation is caused by pressure 
changes because the watery tissues are only slightly compressible. 



30 



The Medium 



Green plants are confined to relatively shallow subsurface depths be- 
cause of their need for light. Many types of animals, on the other 
hand, display a very great vertical range in their distribution. 

Several species of invertebrates are found at depths extending from 
near the surface in the littoral zone down to 4000 m or even 5000 m 
in the abyssobenthic zone (Fig. 2.4). Although the individuals of 
these bottom-living forms do not travel far, the species as a whole 
have become adapted to this great vertical range over a period of 



Neritic 



Oceanic - 




FIG. 2.4. The chief zones of the marine environment. The division between the 
neritic and oceanic provinces occurs at the edge of the continental shelf where 
depth is about 200 m. The lower limit of the archibenthic zone occurs between 
800 and 1100 m. The littoral zone forms the upper part of the neritic benthic 
zone and usually receives strong wave and current action and sufficient light for 
plant growth. The depth of its lower limit is variable but is often in the neighbor- 
hood of 40 to 60 m. These divisions also apply in a general way to lakes. 
(Modified from The Oceans by Sverdrup et al., 1942, copyright Prentice-Hall, 

Inc., N. Y.) 

time. In certain other kinds of marine animals the individuals are 
known to change level over considerable distances and hence are 
able to withstand correspondingly great pressure changes. Some 
species of fish move downward as much as 400 m during the day and 
swim up again to their former level each night, thus subjecting them- 
selves twice daily to a pressure change of 40 atmospheres. Certain 
small planktonic Crustacea similarly carry out diurnal vertical migra- 
tions of 200 m to possibly 600 m in amplitude (Waterman, Nunne- 
macher, Chace, and Clarke, 1939). 

The foregoing is not to imply that most species of marine organisms 
do not have definite vertical limits to their distribution. Many species 



Pressure Increase with Depth 31 

are confined to relatively narrow zones, but these restrictions of range 
are probably due primarily to other factors, such as temperature, 
light, or food. Nevertheless very great changes in pressure have been 
shown experimentally to alter the rates of certain physiological reac- 
tions; and a variety of invertebrate animals, fishes without swim 
bladders, and bacteria are inactivated, or killed, when subjected in 
pressure chambers to several hundred atmospheres, although the ef- 
fect is much reduced if the temperature remains nearly constant 
( Zobell and Oppenheimer, 1950 ) . Thus, although moderate pressure 
changes do not ordinarily harm such organisms, great changes may 
exert certain subtle influences on their life processes. Whether or 
not such physiological action of pressure actually limits the vertical 
range of aquatic organisms in their natural habitats has not yet been 
ascertained. 

For animals with air-filled cavities, such as fish possessing swim 
bladders and diving birds and mammals, the rapid increase in pres- 
sure with depth in the aquatic environment is a serious matter. 
The swim bladder of a fish supplies buoyancy, and a fish with this air 
cavity is similar in its flotation to the Cartesian diver of the physicists. 
When the fish moves downward, the bladder is compressed, and when 
it moves toward the surface, the bladder expands. Gas must be re- 
moved from the swim bladder, or added to it, in order for the fish to 
maintain control of its buoyancy equilibrium. If the fish moves up- 
ward so fast that gas cannot be removed from the swim bladder at a 
rate sufficient to compensate for the reduction in pressure, the con- 
tained gas will continue to expand and the fish will rise toward the 
surface at an accelerating rate. If movement toward the surface con- 
tinues, the swim bladder will eventually burst, and the expanding 
gas, now in the body cavity, will force the stomach to protrude from 
the mouth, the intestine from the anus, and the eyes from their 
sockets. Presumably, under ordinary conditions, an early stage of in- 
ternal volume change stimulates the fish to return to its former level. 
The pressure factor thus definitely limits the vertical range of fish 
with swim bladders as well as the speed with which they can move 
from one depth to another (Jones, 1952). 

For diving mammals and birds the problem of breathing is added 
to that of the increased pressure. When a human diver descends into 
the water in a flexible diving suit he must withstand the increased 
pressure, but air is supplied to him through a hose or by means of an 
"aqualung." Pressure alone prevents the diver from descending more 
than 100 m or so. Whales, seals, and diving birds are forced to go 
without a renewal of oxygen during the period of their dive. Whales 



32 The Medium 

apparently withstand the great pressures by allowing their lungs to 
be completely flattened and the air that was in them to be forced 
into the strong, boxlike larynx. How a mammal swimming vigorously 
can get along for more than a few minutes without a renewal of its 
oxygen supply has been a matter for speculation for generations. 
Evidence obtained primarily from the study of seals indicates that 
these diving mammals can store an increased amount of oxygen in 
their tissues and that this oxygen is reserved for the brain, heart, and 
other vital organs by cutting off the circulation to other parts of the 
body. The muscles build up an oxygen debt that is paid off when 
the animal surfaces again ( Scholander, 1940 ) . Too rapid ascent may 
result in the formation of gas bubbles in the blood, but the fact that 
the whale, unlike the human diver, has only one lungful of air during 
the dive presumably reduces the danger from absorbed nitrogen. 

The depth and duration of the whale's dive have been hotly con- 
tested by captains of whaling ships and others for many years. It 
seems probable that whales dive to 200 or 400 m regularly. Seemingly 
indisputable evidence for an even greater dive was furnished by the 
discovery of a sperm whale that had become entangled at a depth of 
about 1000 m in a submarine cable running between two of the Carib- 
bean Islands. At this depth the pressure is about 100 atmospheres 
(or over 100 kg per sq cm), and evidently the mechanism of the whale 
is adapted to withstand pressures as great as this. Whales ordinarily 
stay submerged for twenty minutes or so, but when harpooned they 
may disappear from the surface for one to two hours. Such dives, 
accompanied as they are by violent swimming, are a striking demon- 
stration of the ability of these animals to remain active for long pe- 
riods without renewal of oxygen. 

SUPPORT AND RESISTANCE TO MOTION 

The difference in density of the two elemental media, air and water, 
also influences the degree to which they provide support and re- 
sistance to motion. Since water has nearly the same density as proto- 
plasm, whereas air is very much less dense, water furnishes much 
more buoyancy than air. 

Effects on Structure and Size 

Since terrestrial organisms are very much heavier than their sur- 
rounding medium, they would tend to collapse from their own weight 
if it were not for special supporting structures. On land only very 



Effects on Structure and Size 33 

small organisms and such animals as earthworms and slugs can main- 
tain their shape without skeletal material of some sort. The woody 
tissue characteristic of the higher plants provides rigidity against the 
force of gravity. The bones and muscles of the larger land animals 
are similarly arranged primarily to provide support (Thompson, 
1942). 

Generally speaking, the weight of an organism tends to increase as 
the cube of its linear dimensions, but the strength of supporting 
columns increases only as the square of the dimensions. As a result 
animals and plants are definitely limited as to size. Since land plants, 
once established, do not require locomotion, they can have a much 
larger amount of rigid supporting tissue than animals. Hence the 
plant kingdom holds the record for size on land. The giant redwoods 
of California (Fig. 1.1) attain heights well over 100 m (record height: 
365 ft), and the trunks alone are estimated to weigh as much as 500 
or 600 tons. In the animal kingdom few modern species attain a size 
as great as 6 or 7 tons, although the dinosaurs of the past were some- 
what larger. Possibly Brontosaurus would have tipped the scales at 
30 or 40 tons, but Brontosaurus came to an unhappy end, no doubt 
in part because of its ungainly size. 

In the water environment, since all parts of the organism tend to be 
buoyed up by the medium, supporting structures may be greatly re- 
duced or entirely lacking. Woody tissue is needed for support by 
few aquatic plants. When an elaborate skeleton is present in aquatic 
animals, it usually occurs for purposes other than support. For many 
crustaceans and mollusks the hard tissues serve primarily as pro- 
tection; in other forms, such as fish, the skeleton is used chiefly for 
the attachment of muscles of propulsion. Many aquatic organisms 
such as the jellyfish have no skeleton at all. It is true that the jellyfish 
is a weak and sluggish organism but the octopus and the giant squid 
are decidedly vigorous, and yet the skeletons of the latter are reduced 
to horny pens and a few cartilages in the head region. An octopus 
kept in an aquarium at the Bermuda Biological Station was so suc- 
cessful in getting out of his tank in spite of a weighted lid that he was 
named Houdini. 

In the plant kingdom even the algae can grow to tremendous sizes. 
The giant kelp Nereocystis, common off the west coast of the United 
States, may grow to a length of more than 35 m, and Macrocystis is 
reported to attain an even greater size. Such plants have no woody 
tissue whatsoever, but the water buoys up the extended parts of the 
organism. Animals in the sea today attain sizes larger than ever 
existed on land. By weighing the parts of a blue whale being cut up 



34 The Medium 

on the deck of a factory ship Hjort (1937) obtained the tabulated 
values, which do not include the blood and viscera. 



Muscles 56 . tons 

Bones 23 . 

Blubber 26. 
Tongue 3 . 

Heart 0.6 



108.6 

If a whale becomes stranded on the beach, the weight of its body 
prevents breathing by crushing the lungs, and its great strength is of 
little avail for getting it back into the water since its muscles of pro- 
pulsion are adapted exclusively for swimming in the open sea. 

Effects on Locomotion through Medium 

Differences in the viscosity, mobility, and inertia of air and water 
have profound effects upon the resistance of the medium to the motion 
of organisms through it. In general the resistance of water is very 
much greater than that of air but the actual value depends upon the 
size and shape of the organism, the viscosity of the medium, and the 
speed of locomotion. The coefficient of viscosity of water is 60 
times that of air at the same temperature. The result is that an im- 
portant resistance to locomotion is felt at very much lower speeds in 
water than in air. Animals whose living depends upon rapid swim- 
ming through water must be thoroughly streamlined. In the mack- 
erel, for example, not only is the body almost perfectly streamlined 
but also the fins fold back into grooves and the surfaces of the eyes 
conform exactly to the contour of the head. 

Another result of this great difference in the resistance of air and 
water is that really high speeds can be attained by animals only in the 
air environment, and even at low speeds very much more effort is re- 
quired to move through water than through air. The speed record 
for the animal kingdom is probably held by the duck hawk, whose 
flight has been clocked at 288 km per hr ( 180 miles per hr ) . Several 
other species of birds can fly at speeds greater than 160 km per hr, 
but no running animal can approach these velocities. The gazelle 
and the antelope are credited with speeds of 96 km per hr, and the 
cheetah can do 112 km per hr (70 miles per hr) over short distances. 
These catlike animals are employed by the natives in Africa to bring 
down antelope for them. The natives steal up as close as possible to 



Passage of Medium through Organism 35 

a herd of antelope in an old Ford and then release the cheetah for 
the last short dash, 

In the aquatic environment fish of the mackerel tribe are the fastest 
swimmers and attain speeds as great as 48 km per hr (30 miles per 
hr). The flying fish has been reported to attain 56 km per hr just 
before its take-off. Anyone who has watched a flying fish, however, 
will remember that in the last moments before the fish leaves the 
water most of its body is in the air, with only the tail sculling violently 
in the surface like an outboard motor. 

Even the method of propulsion is controlled to a large extent by 
the elemental difference in the nature of the air and water media. 
Since the density, viscosity, and inertia of air are so low, most animals 
cannot use the air alone for propulsion but must obtain a purchase 
on the earth's surface. Only birds, insects, and a few other animals 
can propel themselves wholly in air. In the aquatic environment, on 
the other hand, the majority of animals swim in the free water, and 
those for which speed is important do not use the substratum for ef- 
fective locomotion. The lobster, for example, pokes around on its 
walking legs, but to make a sudden dash it uses swift strokes of its 
tail, letting its legs leave the bottom entirely. Because of the rela- 
tively great inertia of the water medium, some animals, such as jelly 
fish and scallops, can propel themselves in one direction by pumping 
water in the opposite direction an early version of "jet propulsion." 
The squid can dart backwards with remarkable rapidity by ejecting 
water from its siphon. 

Passage of Medium through Organism 

Sometimes the medium must move through the organism instead 
of, or in addition to, the movement of the organism through the 
medium. Because of the great mobility of air, this medium can move 
in and out of the cavities of an animal or plant with relative ease, but 
water does not circulate as freely. Special adaptations are required 
to carry water to the tops of trees, including root pressure, transpira- 
tion, and the tensile strength of fine water columns. The movement 
of the water through the tracheids of the plant is relatively slow. 
Even in aquatic plants direct water exchange by osmosis or colloidal 
imbibition requires considerable physical force. 

Although large expenditure of work is necessary, some aquatic ani- 
mals do succeed in causing water to flow through relatively simple 
respiratory or feeding chambers. But water could not possibly be 
pumped into and out of a finely branched system of tubules like the 



36 The Medium 

tracheae of the insects with sufficient speed to meet respiratory 
needs. No doubt partly for this reason relatively few kinds of insects 
have succeeded in establishing themselves in the aquatic environment. 
Most of these remain in the water for only a portion of their lives 
usually the larval stage. Insects living completely submerged possess 
tracheal gills or other special devices for obtaining oxygen without 
taking water into the tracheae. Many fresh-water species are adapted 
to come to the surface for air. In the marine environment Halobates 
lives on the surface of the ocean as a "water strider," and several 
dipterans inhabit shallow areas as larvae, but only one insect, the 
midge Pontomyia natans, is known to complete its entire life cycle 
submerged in sea water. 

Existence of Plankton 

The relatively high density of the water medium not only tends 
to buoy up parts of the body but also in some instances supports the 
whole body and thus allows certain organisms to float at various 
depths in the free water. This fact makes possible the existence of 
plankton plants and animals that live suspended in the ocean and 
inland water bodies and that drift about either because they are non- 
motile or because they are too small or too weak to swim effectively 
against the currents (Figs. 2.5 and 2.6). The term plankton is de- 
rived from a Greek word meaning "wanderer," and many organisms 
in this category, or plankters, spend their whole lives drifting in 
the water. Both animal and plant plankton is found in practically all 
natural waters, frequently in enormous abundance and variety. 
Other categories of life in the aquatic environment are the benthos, 
which consists of the organisms living on or in the bottom material, 
and the nekton, which is composed of the strong-swimming animals. 
The benthos and the nekton have their counterparts on land, but the 
permanent plankton represents an important category of life that is 
totally absent from the air environment. 

Certain planktonic animals and plants live permanently suspended 
in the water by actual flotation. This method of support is not pos- 
sible in the air. Pollen grains, seeds, and spores, commonly spoken 
of as "floating" in the air, are not actually doing so, but are sinking 
at a slow rateoften retarded by various feathery structures. In the 
water actual flotation is possible for forms that contain air cavities or 
light materials such as fats or oils. The brown alga Sargassum is pro- 
vided with gas-filled bladders, and the Portuguese man-of-war, a 
siphonophore, has a pneumatic "sail." Many fish eggs float by virtue 



Existence of Plankton 37 

of droplets of oil. Some pelagic diatoms completely counterbalance 
the weight of their siliceous shells by means of a cell sap which is 
lighter than water so that these organisms have no tendency to sink. 

In addition to the truly floating forms many other kinds of plants 
and animals sink so slowly that they are able to lead a planktonic 






*-* . / 1- ;;p** ;> : 

k _ I / , f^ 5 -9 .1 




Bigelow, 1926 

FIG. 2.5. Marine phytoplankton. Photomicrograph of several common genera of 
oceanic diatoms. The largest cell is approximately 0.05 mm in diameter. 

life. In the air environment the sinking rate of the smallest organ- 
isms, such as bacterial spores, is extremely slow, but eventually all 
particles settle out. In the water, however, the sinking rate of some 
organisms, including many multicellular animals, is so retarded that 
a small amount of swimming allows the organisms to maintain their 
position. In other instances the amount of sinking may be inconse- 
quential before the organism is brought up to the surface again by 
vertical currents. 



38 



The Medium 



Sinking rate is also controlled in part by size and shape. It might 
be said that one of the simplest ways of reducing sinking speed is to 
be small because with reduction in volume the surface and hence 
surface friction becomes relatively greater. Any departure from the 
spherical shape increases the surface of an organism. Unicellular 
forms like the pelagic Globigerina which are bristling with spines 




Vigelow, 

FIG. 2,6. Marine zooplankton, Photomicrograph of common copepods, chaetog- 
naths, and medusae. The euphausiid "shrimp" is approximately 2.5 cm (1 in.) 

long. 

graphically illustrate this fact (Fig. 2.7). Most multicellular animals 
that live a planktonic life also are liberally provided with long an- 
tennae, spines, and bristles of a great variety of shapes. The effect 
of these changes in the morphology of planktonic organisms, com- 
bined with differences in density, in reducing the sinking rate is il- 
lustrated in Table 1. 



Existence of Plankton 



39 




./wurray ana njon, 1x11, copyngni jnacminan & Co. 

FIG. 2.7. Globigerina bulloides, a planktonic protozoan belonging to the order 
Foraminifera. The largest sphere of the shell is about 0.5 mm in diameter. 

TABLE 1 

COMPARISON OF SINKING RATES OF THREE TYPES OF PLANKTON ORGANISMS 
WITH THOSE OF SPHERICAL SAND AND SILT PARTICLES 



Sand grain 
Copepod (Calanus) 
Silt particle 
Diatom (Nitzschia) 
Bacteria 



Length or 

Diameter 

(millimeters) 

1.0 

3.0 

0.01 

0.020 

0.001 



Sinking Rate 
(meters per day) 
8600. 
576. 
14.5 
0.050 
0.132 



Since the density of the water environment is closely similar to that 
of organisms living in it, the reader will not be surprised to learn that 
slight changes in the former often have important consequences in 



40 The Medium 

the vertical distribution of plankton. Differences in the density of 
fresh water and sea water at three temperatures within the biological 
range are shown in Table 2. As a result of these density differences, 

TABLE 2 
SAMPLE VALUES FOB THE DENSITY OF WATER AT DIFFERENT TEMPERATURES 

Fresh Water Sea Water (salinity 35%c) 

Temperature C (g/cc) (g/cc) 

0.999 1.0281 

4 1.000 1.0278 

SO 0.995 1.0217 

relatively sharp stratification of water masses may come into being in 
lakes and in oceanic areas. If other influences do not interfere, 
warmer water tends to remain over colder water, and fresher water 
tends to float on top of the more saline. 




Murray and Hjort, 1911, copyright Macmiltan & Co. 

FIG. 2.8. Calocalanus pavo, a tropical planktonic copepod. The length of the 
crustacean's body is about 2.5 mm. 

Although the differences in density shown in Table 2 may seem 
very small, they are sufficient to influence profoundly the circulation 
of water in lakes and in the ocean, and to make the difference be- 
tween floating and sinking for planktonic organisms. Plankton some- 
times tends to sink through the upper stratum of less dense water, 
but to stop sinking when it reaches a stratum of higher density below. 



Existence of Plankton 41 

In such a situation plankton and detritus accumulate and form a 
"false bottom" that provides a potentially rich feeding zone. 

Important changes in viscosity also occur as temperature is al- 
tered. At 25C the viscosity of water is only half that at 0C. Since 
at higher temperatures the effect of the decreased viscosity is added 
to that of the lower density, the rate of sinking tends to be greater. 
Probably as an adaptation to this difference through natural selection 
many planktonic organisms in warmer waters are more profusely pro- 
vided with bristles and other feathery structures tending to retard 
sinking (Fig. 2.8). Certain planktonic diatoms and dinoflagellates 
also show a graded difference in the length of spines in tropical and 




July Sept. Oct. Dec. Mar. Apr. June July 




Redrawn from Wesenberg-Lund, 191 '0 

FIG. 2.9. Cyclomorphosis in Daphnia cucullata (upper) and Bosmina coregoni 
(lower), showing seasonal changes in body form. 

polar areas. Whether such geographical variants should be divided 
into separate species may not be settled until controlled culturing ex- 
periments are undertaken. 

In some plankters the body form changes with the season of the 
year. This phenomenon, known as cyclornorphosis, is strikingly ex- 
hibited by the increased size of the "helmet" and length of spines 
in certain Cladocera during warm months ( Fig. 2.9 ) . These changes 
produce increase in body surface and were originally believed to be 
entirely an adaptation to the lower density and viscosity of warmer 
water, but subsequent work has shown that factors other than tem- 
perature also influence body form (Brooks, 1946). In all such re- 
lationships the ecologist must distinguish between actual causal fac- 
tors and possible beneficial results. For a further discussion of 



42 The Medium 

plankton in relation to flotation the reader may refer to Sverdrup 
et al. (1942) and Welch (1952). 



TRANSPORTATION BY MEDIUM 

Another significant action of the medium is its action in providing 
transportation for plants and animals. Certain requirements such as 
light can often reach the organism without the movement of either 
the organism or the medium. Most necessities, however; are not 
adequately provided for unless either the organism or the medium 
moves. Put in simplest terms the needs for mobility are: (1) to 
provide materials for metabolism and growth; (2) to remove waste 
products; (3) to bring male and female elements together; (4) to 
distribute progeny; (5) to avoid unfavorable conditions. 

Either the organism must be able to forage and to distribute itself 
effectively, or else some mobile agent must be available. Fortunately, 
both media are mobile, but their potentialities for providing transpor- 
tation differ considerably. Air moves much more readily and much 
faster than water, but water can carry heavier objects in suspension. 
The composition of air remains relatively constant, but the water 
medium contains widely varying amounts of ecologically important 
substances. 

Movement of the air medium ranges from local wind currents of 
beneficial nature to violent and destructive gales. Animals and 
plants in some areas are subject to strong winds which vary in direc- 
tion from day to day, whereas in other regions, such as the trade- 
wind belts, the wind blows almost constantly from one quarter with 
corresponding unidirectional influence. Air movement also indirectly 
affects other factors such as temperature, rainfall, and evaporation. 
The circulation of the atmosphere controls the weather and provides 
for the transport of moisture from the sea to the land. 

Movement of the water medium similarly varies from small-scale, 
sporadic circulation, generated by local wind and waves, to larger 
and stronger currents. Plants and animals living in ponds and lakes 
are regularly subject to wind-driven currents. In fresh-water streams 
the one-way transport of water is a matter of vital concern to all the 
inhabitants. Along the sea coast, on the other hand, the tidal cur- 
rents are characteristically reversible or oscillatory. Farther offshore 
the permanent currents are encountered, and these are parts of the 
great current systems of the ocean which include a rotary circulation 
in each of the ocean basins (Fig. 2.10). Two well-known oceanic 
currents of great ecological significance in the northern hemisphere 




43 



44 The Medium 

are the Gulf Stream, which carries water of tropical origin northward 
along the east coast of the United States and thence northeastward 
toward northern Europe, and the Japanese Current, which brings 
relatively warm water to the Aleutians and the Alaskan coast. 

Sessile Existence 

Since both air and water are practically always on the move, the 
possibility exists that animals and plants might live a lazy existence, 
remaining in one place and letting the medium bring their needs to 
them. The desirable features of such a tranquil life might appeal 
to many of us. But since air can carry only the smallest and lightest 
particles, no free-living animal on land can obtain sufficient solid food 
by air transport. There are no completely sessile animals in the ter- 
restrial environment. A few land animals, such as the spider and 
the ant lion, lie in wait for the prey which flies or crawls to their 
traps, and other forms, such as the dung beetle and the wood borer, 
live within their food material, but even for these animals some loco- 
motion is necessary. 

The needs of land plants, on the other hand, are such that most of 
them can be brought by mobile agents. The carbon dioxide and 
oxygen exchange of the plant is readily taken care of by the movement 
of the air. Mineral nutrients needed by the plant are also carried 
to it, but in most situations these materials are brought by the soil 
water so that we are really dealing with a special case of the mobility 
of water. However, certain epiphytes, such as the bromeliads and 
Spanish mosses, live in the crotches of trees or even on telephone wires 
where absolutely all their needs reach them by means of air transport. 
These "air plants" obtain mineral nutrients from rain water and from 
dust particles which lodge in their crevices. For many species the 
wind plays an essential role also in reproduction and in distribution. 

In the water environment the transport by the medium of the needs 
of both plants and animals is quite an ordinary occurrence. Most 
multicellular plants in aquatic habitats lead a completely sessile exist- 
ence, allowing the water to bring them the oxygen, carbon dioxide, 
and food materials that they require. The algae absorb their nutrients 
directly from the free water, but the vascular plants generally obtain 
these materials through their roots from the water in the mud. 

Large numbers of aquatic animals enjoy a sedentary life. Many 
groups of sessile animals, such as the sponges, coelenterates, bivalves, 
and barnacles, are extremely abundant in the water environment, but 
are entirely unrepresented on land. These forms can remain per- 



Transport by Air 45 

mancntly attached to the bottom, allowing food particles to be brought 
to them by the water. Many of these animals are characterized by 
radial symmetry; since food may be brought from any direction, a 
radial arrangement of feeding appendages around the animal's mouth 
is efficient. Only when locomotion is required is it particularly de- 
sirable to have a head and a tail end with attendant bilateral sym- 
metry. Colonial existence is also possible among sessile organisms 
but would be very clumsy for forms that require active locomotion. 

In aquatic plants and animals the male reproductive cells are carried 
to the egg cells by water movement, or both are discharged into the 
surrounding medium where currents bring them together to accom- 
plish fertilization. The young of sessile organisms are effectively re- 
moved from the neighborhood of the adults during the larval stage. 
The planktonic larvae, carried far and wide by the currents, ac- 
complish the dispersal of the species and the colonization of new areas. 

Many marine and fresh- water animals grow more effectively in a 
current than in quiet water, and some forms can live only where the 
water is moving rapidly. Certain caddis fly larvae ( Hydropsyche ) 
living on stream bottoms construct funnel-shaped nets with openings 
upstearm into which food particles drift. The larva of the mayfly 
(Chirotenetes) braces itself on the bottom with its head upstream and 
spreads its hairy prothoracic legs like a net with the result that par- 
ticles of food in the flowing water are funneled into the animal's mouth 
(Morgan, 1930). The term current demand has been used for the 
dependence of certain forms, particularly stream forms, on a move- 
ment of the water, but in sqme situations the precise reason that a 
current is necessary is not clear. The larva of the black fly (Sim- 
ulium), for example, will not develop in quiet water even though 
plenty of oxygen and other obvious needs are available. 

Distribution by Medium 

If the needs are not brought to the organism, the organism must 
go after them either by its own locomotion or by hooking a ride on 
something else. The transportation service of the environment is of 
particular value to the seeds or larvae of sessile organisms and for 
those motile organisms that are small or feeble and hence would be 
very slow in getting around under their own steam. 

Transport by Air. No organism can live permanently floating in 
the air, but this medium is often useful in the sporadic and intermittent 
transport of terrestrial organisms ( Wolfenbarger, 1946). The smallest 
forms, such as the bacteria, can be transported effectively by even a 



46 The Medium 

slight movement of air. ZoBell calculated that if bacteria were re- 
leased at a height of 33 m when the wind velocity was as low as 16 
km per hr (10 miles per hr), the microbes would be carried 4800 km 
before they reached the ground. The heights to which small or- 
ganisms may be carried by turbulence is illustrated by a record of 
fungal spores collected on plates exposed from an airplane at an 
altitude of 3600 m. The rapid transport by air of spores causing 
diseases in both plants and animals represents an ever-present threat 
to economically important populations. New strains of wheat rust, 
for example, seem to be brought in by the wind almost as fast as 
resistant types of wheat are developed. 

Many of the earth's most abundant types of higher plants, such as 
the conifers, depend largely upon wind pollination. Pollen grains are 
regularly transported hundreds of miles by the wind, and pollen has 
been detected in the air over the ocean more than 1000 miles from 
land. The seeds of orchids and of certain other groups are so small 
that they are carried by the wind almost as effectively as pollen grains 
and spores. Hairy structures, such as the familiar "parachutes" of 
the dandelion, make possible the wind transport of larger seeds over 
considerable distances, and wings, bladders, etc., such as those of the 
maple and elm, enable many of the heavier seeds of these trees to be 
blown far enough from the parent plants to avoid immediate com- 
petition (Siggins, 1933). In the grasslands and desert regions yet 
another bizarre method for transport is encountered in the tumble- 
weed, the spherical upper portion of which breaks off and rolls for 
miles before the wind, scattering the seeds as it goes. 

Wind also plays a significant role in the distribution of animals, par- 
ticularly of insects. The wings of flying insects provide the lift, and 
even ordinary winds provide horizontal translocation which may carry 
them very great distances (Gislen, 1948). Elton (1939) found cer- 
tain aphids and flies alive over Spitzbergen after a wind drift over the 
ocean of about 1300 km. Stronger winds carry flightless insects, 
spiders, and other small invertebrates in either the active or the 
encysted condition. Strong prevailing or seasonal winds in certain 
situations tend to exert a regular influence on the distribution of in- 
sects. Along the northern shore of the Gulf of Mexico, human in- 
habitants are pestered by mosquitoes regularly blown many kilom- 
eters inland from the salt marshes by the onshore breeze, and Garrett- 
Jones (1950) reported mass wind-borne invasions of areas as much as 

47 km from the mosquitoes' breeding places in Egypt. As compared 
with oceanic currents, however, winds tend to be irregular, and in- 
sects, as well as other flying land animals, can often migrate against 



Transport by Air 47 

the wind, as has been shown to be true for the monarch butterfly 
(Williams et al,, 1942). Consequently, regular transport by the 
medium generally plays a far smaller part in the lives of land animals 
than in the lives of the denizens of the sea. 

Occasionally the geographical range of an insect pest is extended 
by wind action. A classical example of such an occurrence is pre- 
sented by the spread of the gypsy moth in New England. This 
foreign insect escaped from cages in which experiments were being 
conducted in the vicinity of Medfield, Mass., in 1869. Since the 
female of the species is flightless, one might suppose that the insect 
would be confined to the immediate neighborhood of its point of 
introduction, or at least would spread very slowly. However, the 
species has a special way of "thumbing a ride" on the wind. The 
newly hatched caterpillars in their first instar are provided with espe- 
cially long hairs. When they crawl to the tops of trees and spin 
long threads, they are soon blown off and are carried considerable 
distances before they reach the ground. The caterpillars climb again 
to the tree tops and the process is repeated. By this means, to the 
detriment of the oak forests, the gypsy moth was spread throughout 
New England within a few years. It even succeeded in crossing Cape 
Cod Bay, a distance of about 40 km. 

The lifting power of the wind during hurricanes and tornadoes is 
well known and provides an exceptional opportunity for the transport 
of larger animals and plants in unexpected directions. Although in 
most regions hurricane winds are rare, over the centuries they may 
nevertheless have made possible the introduction of new species to 
islands and other locations which would ordinarily be inaccessible to 
the forms concerned. If the new arrivals become established and if 
they are predatory or infectious or if they compete successfully, though 
passively, with the native species, they may completely upset the eco- 
logical adjustments of the existing community. 

It has long been recognized that the fauna of the Greater Antilles in 
the West Indies has been derived in the evolutionary sense principally 
from the Central American fauna. No convincing evidence has been 
brought forward, however, that land bridges ever connected these 
islands with the mainland of Central America. How the amphibians, 
rodents, snakes, and other species of this general size ever reached the 
islands remained a mystery until Darlington (1938) pointed out the 
possibility of transport by the hurricanes which so frequently cross 
this area. 

The exceptional lifting power of certain atmospheric disturbances 
is illustrated by the "rain of fishes" which occurred in 1947 at Marks- 



48 The Medium 

ville, La., and is described by an eye witness (Bajkov, 1949). On the 
morning of October 23, fish ranging between 5 and 23 cm in length 
fell on the streets and in the yards, mystifying the citizens of the town. 
There were areas along the main street in which the abundance of fish 
averaged one to every square meter. The fish belonged to the fresh- 
water species native to the local ponds. Although no large wind 
storm occurred, numerous small "devil duster" tornadoes had been 
noticed in the area. At Marksville the majority of fish were dead 
when picked up from the ground, but it is perfectly possible that 
many of them would have survived had they fallen in water. Stock- 
ing of remote ponds by dumping fish from airplanes has often been 
successfully accomplished. Transport by local violent air currents 
may therefore explain the introduction of fish into land-locked ponds 
or lake systems not connected with other fresh-water areas in which 
the species occurred. 

Transport by Water. Since water has a much greater buoyant 
effect than air, transport by the water medium is of vital concern to 
many more kinds of plants and animals than transport by air. Trans- 
port by currents plays an important role in distribution in both inland 
waters and the sea. However, we find the fresh-water organisms do 
not utilize this transportation system to nearly as great an extent as 
marine organisms. This difference may be partially explained by the 
fact that fresh water is less dense and hence less buoyant. Also, in- 
land waters are less permanent. Since fresh-water bodies often dry 
up, or freeze, their currents cannot always be relied upon for trans- 
portation. 

Correlated with the foregoing facts, we find that many types of 
animals whose marine species have free-living larvae are represented 
in fresh water by species with a much shorter larval life. In fresh 
water there are many more species in which the eggs or young remain 
attached to the adult, as in the copepod, Cyclops, and the crayfish, or 
are retained within a brood pouch as in the Cladocera (Fig. 2.11). 

If stream animals make use of the water transport system, they must 
have some method for getting back upstream again. Some of the 
devices serving this purpose are most intriguing. The larvae of 
mussels are provided with special hooks with which they attach 
themselves to the gills of fish. Some of the fish that are thus parasi- 
tized eventually wander upstream where the maturing larvae drop off 
and metamorphose into adult mussels. Other stream forms resort to 
the formation of resistant spores that may be carried by birds or blown 
by the wind. 

Oysters and other benthonic animals remain established in tidal 



Transport by Water 49 

rivers and estuaries although the planktonic larvae might be expected 
to be carried away since the net water movement is always toward 
the sea ( Ketchum, 1951 ) . In some situations the denser, more saline 
water near the bottom tends to move predominantly into the estuary 
whereas the greater flow in the surface strata is seaward. Observa- 
tions in certain estuaries indicate that the older oyster larvae tend to 
drop to the bottom on the ebb tide and to rise into the water on the 




Single winter egg in ephippium 



Redrawn from Needham and Lloyd, 1937 
FIG. 2.11. Ceriodaphnia reticulata showing eggs developing in the brood pouch. 



flood tide (Carriker, 1951). In this manner a sufficient number of 
young oysters work their way upstream to repopulate the upper 
regions of the estuary. In barnacles the problem is solved by the fact 
that, although the younger larvae are found in the upper water layers, 
the larvae approaching the setting stage tend to concentrate at deeper 
levels where the net drift carries them up the estuary (Bousfield, 
1954). 



50 The Medium 

In the ocean many more groups are influenced by the transporting 
action of the water. This affects not only the varied permanent 
planktonic population but also the planktonic larvae of the benthos 
and of the nekton. Species having planktonic larvae are much more 
numerous in the marine environment than in fresh water. The plank- 
tonic life of marine larvae also tends to be longer, often with many 
more stages. Certain euphausiid crustaceans, for example, have five 
larval stages including about twenty moults before reaching the adult 
condition. 

So characteristic are the planktonic forms of many water masses 
that they are sometimes used in tracing the currents. Members of the 
permanent plankton which live in specific water masses must go 
wherever the water goes, like Mary's lamb. Such species are termed 
current indicators. Russell (1939) has shown, for example, that the 
"Channel water" ordinarily occupying the English Channel is popu- 
lated by a plankton community of which the chaetognath, Sagitta 
setosa, is a characteristic member. Occasionally, however, a mass of 
"mixed oceanic and coastal water" moves into the Channel from the 
region south of Ireland, and its presence is revealed by the abundance 
of another species, Sagitta elegans. A third species, Sagitta serrato- 
dentata, is an indicator for the "pure oceanic water" which is found 
to the west of the water mass tagged by S. elegans. 

The Gulf weed Sargassum, which can be seen by any traveler 
crossing the southern part of the North Atlantic ocean, is an indicator 
for water of tropical origin. This plant begins life attached to the 
bottom around certain of the West Indian Islands (Parr, 1939). It is 
torn up by storms and drifts out to sea where it continues to grow 
vegetatively for years. Masses of Sargassum clumped together gave 
rise in the past to the legend of the Sargasso Sea where ships were 
supposed to become hopelessly entangled in the seaweed. You will 
remember that, in crossing the Atlantic, Columbus was encouraged 
by the presence of drifting seaweed to keep on, believing that he had 
seen an indisputable sign of the proximity of land. It is fortunate 
that Columbus did not realize that Sargassum may be carried hun- 
dreds or even thousands of miles from shore by the currents of the 
ocean. 

The matter of transport by ocean currents should not be left without 
mention of the classical example of the eel. For generations the 
people of Europe wondered where eels came from. Young elvers 
ascended their rivers in the spring, and mature eels left their rivers 
in the fall after several years' life in fresh water. No one knew where 
or when the eels spawned. Finally, it was discovered that a small, 



Transport by Water 51 

flat planktonic organism, long thought to be another species entirely, 
was the larva of the eel. The Danish marine biologist, Johannes 
Schmidt, then set out to find the spawning ground from which the 
larvae came. Little did he know, when he began, that his search 
would require many years of work and thousands of miles of explora- 
tion before he answered the question. By plotting the occurrence of 
smaller and smaller larvae, Schmidt (1925) gradually traced the drift 
of the larvae back to its point of origin southeast of Bermuda (Fig. 
2.12). Here the eels spawn apparently deep in the water, since no 



40* 








100* 90 80 7Q P 60* 60 40 30 20 



Redrawn from Schmidt, 1925 

FIG. 2.12, Migrations of the eel. Dotted lines indicate drift of larvae from breed- 
ing areas of American eel ( A ) and European eel ( E ) . Solid lines indicate return 
migration of the adult eels. 

one has seea them. The newly hatched larvae drift into the Gulf 
Stream and after three years are carried to the European shore, 
where they metamorphose into elvers and enter the rivers. The ma- 
ture eels make the return trip under their own locomotion, although 
how they find their way across two or three thousand miles of ocean 
is a complete mystery. 



52 The Medium 

The American species of eel breeds in an area that overlaps the 
spawning zone of the European eel. The Gulf Stream circulation 
similarly plays an essential role in carrying its larvae northward. 
Nevertheless, no record exists of European eels entering an American 
river, or of American eels being found in Europe. The metamorphosis 
of the American eel takes place at the end of one year when it is 
opposite the American shore. At this time it is ready to respond to 
some influence which orients it toward the mouths of the rivers. 
However, its European cousin, mingled with it in the plankton, is not 
yet responsive to shore influences and continues drifting with the Gulf 
Stream water until it reaches the European coast. The eventual ar- 
rival of the two species of eel on opposite sides of the Atlantic is a 
spectacular illustration of the integration of the action of the environ- 
ment with genetic differences in the timing of responses. 

Harmful Transport 

In our discussion of the favorable results of transport by the medium 
in some natural situations we must not lose sight of the fact that this 
influence, like most others, may also produce harmful effects under 
different circumstances. Some terrestrial organisms benefit by the 
distribution provided by the wind, but a great many others are carried 
into regions where they cannot possibly survive. Many land animals 
and plants are blown out to sea or over lakes, where they fall into 
the water. Insects and birds permanently established in windy 
regions have been forced to develop reactions that protect them from 
being blown away. The insect fauna of islands and mountain 
regions frequently includes a disproportionate number of wingless 
species. Although in some circumscribed situations flightless forms 
may have survived chiefly from lack of need of flight (Darlington, 
1943), it is probable that certain flying species have been positively 
eliminated from many exposed islands by wind action. 

Inhabitants of rapid streams are frequently swept down into slug- 
gish rivers or into lakes in which conditions are unfavorable for them. 
Fresh-water forms are carried into the ocean, where usually they are 
quickly killed by the change in salinity. Similar harmful transport 
occurs in the marine environment. All the Arctic organisms that are 
carried southward by the Labrador Current are killed when the water 
in which they are living is mixed with warmer water in the vicinity 
of the Grand Banks. All tropical phytoplankton and zooplankton 
which are swept by eddies out of the northern edge of the Gulf Stream 
succumb as a result of excessive cold. 



Abrasive Action of Medium 53 

Other examples of the destructive action of currents may be on a 
smaller scale but of considerable economic importance. In many 
coastal regions the planktonic larvae of cod, haddock, and other com- 
mercially important fish are normally carried by currents from the 
areas where they are spawned to offshore banks suitable for the feed- 
ing of the juvenile stages when they take to the bottom. There is 
evidence that occasionally unusual eddies carry the young stages 
beyond the banks into the open ocean. When the developing fish 
are ready for bottom life, they find no suitable bottom withia reach, 
and whole populations may perish as a result. A practical applica- 
tion for an understanding of these ecological relations is involved 
in the intelligent establishment of hatcheries both on the coast and 
inland. Before large amounts of the taxpayer's money are spent, 
some assurance should be gained that the young fish released will be 
carried by the existing currents to areas suitable for their further 
development. 

ABRASIVE ACTION OF MEDIUM 

The abrasive action of the medium and of the material carried 
by it, sometimes referred to as "molar" action, is another aspect of the 
ecological influence of the medium. If the medium is air, this action 
means the mechanical force of the wind and the grinding action of 
sand, dust, snow, and other materials driven by it. In the water, even 
stronger abrasive action is produced by waves, currents, and particu- 
larly by stones, sand, ice, and the like, carried by the water. 

Even the wind by itself can influence the growth form of plants in 
exposed regions. The buttresses at the base of the ceiba trees in the 
flat country of Cuba have been shown to develop to the greatest extent 
in directions tending to support the trunk against the most frequent 
winds (Fig. 2.13). In the Texas Panhandle it is said that on a normal 
day a man can expectorate a mile and a quarter! The sand carried 
by winds in such exposed areas produces an abrasive effect which can 
be resisted only by plants with tough cuticle like the cacti and many 
grasses. In mountain regions the amount of strong wind to which the 
vegetation may be subjected is not always appreciated. For example, 
on Mt. Washington, only 1920 in high, the weather station recorded 
a wind velocity of more than 120 km per hr (75 miles per hr) on 
85 days between October, 1940, and March, 1941. The average wind 
speed for the period was 61 km per hr, with a maximum of 219 km 
per hr. The all-time high for Mt. Washington was reached during 



54 



The Medium 




Photo from J. H. Whh 

FIG. 2.13. Buttresses at the base of a ceiba tree near Soledad, Cuba. The great 

leverage action of the wind, which the buttresses help to resist, can be imagined 

from the height and size of a similar tree seen in the left background. 

the early part of the hurricane of 1938 when the anemometer indicated 
a wind velocity of 343 km per hr. The instrument then blew away. 
The continued pressure of even moderate winds blowing pre- 
dominantly from one direction frequently produces a training action 
on the branches of the trees. In addition, branches whipped about 
by the wind on the exposed side of the trees knock off one another's 



Abrasive Action of Medium 55 

growing buds. These effects, augmented by excessive evaporation on 
the windward side, often produce extremely asymmetrical growth, 
"Flag-form" trees with the upper branches restricted to one side of 
the trunk are commonly found on exposed mountain ridges. Groves 
of trees in windy lowlands are often similarly wind-trained, the more 
protected individuals growing many times higher than those on the 
exposed side. The outline of such a grove presents a smoothly rising 
contour (Fig. 2.14). Along the sea coast the harmful action of salt 



V t yppf 

rS P^r * , 

, t /^ . /,'., 'fpfi^^p^ 1 ''* 

( > n '^j^\^fe^t^V': 




FIG. 2.14. Live oaks on wind-swept shore at Morehead City, N. C. Wind action 
has stunted the trees on the exposed side of the grove at the left. 

spray is added to the other effects of the wind in progressively re- 
stricting growth toward the exposed beach and in causing a zonation 
of the species present (Costing and Billings, 1942). At high eleva- 
tions in the mountains low temperatures act with the severe winds to 
produce the familiar dwarfing and gnarling of the vegetation. The 
combined effect of these ecological factors often limits the growth of 
trees to a sprawling mat only a few centimeters high near the summit, 
whereas in the protected valleys trees of the same species grow to 
heights of over 15 m. 

In the water medium abrasive action is an even more serious in- 
fluence with which the organisms must contend. Only strongly at- 



56 The Medium 

tached plants and those animals with streamlined forms and special 
hooks, sucking discs, or other devices for clinging to the bottom can 
exist in turbulent mountain streams (Nielsen, 1950; Welch, 1952, 
Ch. 17). Currents of moderate velocity, such as are encountered in 
the lower reaches of streams and along the shores of lakes and of the 
ocean, produce a force that must be resisted or avoided by the animals 
and plants attempting to maintain a foothold in such habitats. Mov- 
ing sand and silt continually threaten to abrade or to smother the 
organisms present. A current with a speed of only 1.4 m per sec 
(2 l / 2 knots), for example, will move stones and gravel up to 2.5 cm in 
diameter and thus would grind up all unprotected forms. 

At the margin of lakes and particularly on the ocean beach the force 
of breaking waves, and of the sand, gravel, or ice carried by the water, 
produces an extremely serious abrasive action. Where exposure to 
such molar action of waves is particularly severe, the shoreline is often 
barren except for especially adapted species. However, in zones of 
very considerable wave action, a surprisingly large number of species 
of plants and animals have developed methods of maintaining them- 
selves. 

Devices for withstanding the serious molar action of the water 
medium are extremely varied. Anyone who has caught and cleaned a 
bass, a scup, or other fish inhabiting turbulent coastal waters knows 
how much protection the tough skin and thick scales provide for the 
surface of the animal. This resistant integument is in sharp contrast 
to the tissue-thin skin of the deep-sea fish. These latter animals living 
deep in midocean have no wave action to contend with, nor are they 
subject to chafing against rocks or gravel by currents. The inverte- 
brates of the shoreline as well as the fish are protected by a resistant 
outer surface. The shells of mollusks and of crustaceans inhabiting 
such situations are characteristically thick and hard. 

During the course of evolution many species in the littoral zone 
have developed holdfasts, cementing organs, suckers, etc., which pre- 
vent their owners from being washed away. Instead of evolving 
with more and more rigid outside surfaces, certain inhabitants of the 
surf zone solved the problem in the opposite direction. The sea- 
weeds, such as the kelp and the rock weeds, are composed of flexible 
tissue with a leathery exterior. These forms give with the surge of 
the waves rather than being built to resist their force. The form 
and structure of these plants present a sharp contrast to the brittle 
nature of the typical land vegetation. Imagine what would happen 
if your flower garden were suddenly transplanted into the middle of 
the breaker zone! 



Abrasive Action of Medium 57 

Sessile animals, of which there are many representatives in the tidal 
zone, sometimes show a difference in growth form according to 
whether the individuals are living in an exposed or a sheltered loca- 
tion. Sponges and tunicates, for example, grow with long pendulous 
processes in quiet water, but, if exposed to strong currents and wave 
action, individuals of the same species grow closely appressed to the 
rocks (Wilson, 1951). 

Certain physiological adaptations also exist for life where currents 
are strong. Many motile animals exhibit a rheotaxis, that is, an 
orientation of their locomotion with respect to the direction of the 
current. The rheotactic reactions of most stream fish is such that 
they swim against the current and thus maintain their position in the 
stream. Interestingly enough, in some species at least, this reaction 
is mediated through the eyes. The fish are stimulated to turn and 
swim until the image of the stream bottom no longer moves across 
the retina. Other fish are stimulated by differential pressure or 
touch. Many stream animals react in such a way as to move out of 
the current and thus reach quiet water or protected eddies. 

Other reactions to the current are even more elaborate. Some 
caddis-fly larvae, for example, alter the shape of their cases and the 
materials of which they are built according to the strength of the 




Modified from Dodds and Hisaw, 1925 

FIG. 2.15. A series of caddis-fly larvae and their cases from quiet and swift 

waters. 



58 The Medium 

current. Larvae living in quiet water build their cases of large and 
irregular material (Fig. 2.15). With increasing strength of current 
the cases become more and more streamlined and are built of finer 
material. In the swiftest streams they are either completely stream- 
lined or else done away with entirely. 

The power of autotomy exhibited by some crabs may have arisen 
partly in response to the dangers of life in turbulent areas. When 
the leg of a crab has been caught or crushed between moving stones 
it can be cast off, thus setting the animal free. After autotomy has 
taken place, regeneration commences and a new leg grows from the 
base of the old leg. 

Emphasis has been placed in the preceding paragraphs on the 
harmful action of fast-running water and breaking waves. There are 
situations, however, in which extreme turbulence may be beneficial. 
Some species of coral grow best where the pounding of the surf is 
the heaviest, because waves remove sediment which in quiet waters 
would accumulate on the coral polyps and tend to smother them. 
Thus stimulated, coral colonies of this type rapidly produce "heads" 
of coral rock at the exposed outer edge of the reef. Coral heads 
reach such a size that they are eventually broken off by the force of 
the waves. The form of the reef is the result of a balance between 
the increased growth due to the turbulence of the surf and the loss 
due to the breaking off of coral material. Here we have another il- 
lustration *of the dynamic balance represented by natural communities 
and their environments. 



3 

The Substratum 



As a factor of the environment the substratum is only slightly less ele- 
mental than the medium. The substratum is the surface upon which 
the organism rests or moves, or the solid material within which it 
lives in whole or in part. Some ecologists have not distinguished be- 
tween medium and substratum. Another possible source of con- 
fusion is the use by bacteriologists of the term "substratum" for the 
nutrient medium used for growing microorganisms. The important 
point is to distinguish between the concepts, and it seems most logical 
to use "medium" exclusively for the material which immediately sur- 
rounds the organism, and "substratum" only for the surfaces or solid 
materials of the environment on which or within which the organism 
lives. 

SIGNIFICANCE OF THE SUBSTRATUM 

The substratum is not inevitable as is the case with medium. Every 
organism has a medium, either air or water, but some organisms can 
do without a substratum. In the aquatic environment the permanent 
plankton and many pelagic fish have no substratum at any time, but 
terrestrial organisms must have a substratum for at least part of their 
lives since no animal or plant can live permanently suspended in the 
air. Another general difference between the medium and the sub- 
stratum is that, whereas the medium is rarely changed from air to 
water, or vice versa, by the activity of the organism, the substratum 
can be profoundly modified by many of the animals and plants which 
live on it or in it. 

Needs Provided by the Substratum 

Fundamental needs of the organism which may be provided by 
the substratum are purchase, attachment, shelter, and nourishment. 

59 



60 The Substratum 

The limitation of growth or of distribution by the inadequacy of the 
substratum in respect to these needs will be discussed in subsequent 
sections of this chapter dealing with the different types of substratum. 
The substratum may also have importance in various special ways. 
For example, the hue and pattern of many animals protect them from 
detection by enemies because they blend with the background ( Cott, 
1940). The color and texture of the substratum are thus essential 
considerations in the operation of protective coloration in nature. 

The larvae of many sessile organisms will not continue their de- 
velopment unless they find a suitable substratum. This fact has many 
important practical applications as for instance in oyster culture. For 
years oystermen have realized that clean, hard surfaces must be avail- 
able in the spawning areas if the oyster larvae, or "spat," are to make 
a successful "set" each year. To insure the presence of a suitable 
substratum the oystermen dump overboard whole boatloads of empty 
shells or other material at a time when ecological conditions are such 
that the oysters of the region are about to spawn. This specially pro- 
vided substratum is known as cultdi, and its presence in sufficient 
abundance at the critical moment for the attachment of the larvae 
is necessary for successful oyster culture. 

Attainment of the Substratum 

The attainment of a proper substratum is crucially important in the 
lives of most plants and animals, and special methods meeting this 
need have developed during the course of evolution. One obvious 
and common procedure is the broadcasting of such great numbers of 
seeds, spores, or larvae in the attaching stage that some of them will 
eventually "fall on fertile soil/' The majority of terrestrial plants 
follow this method, and, although only an extremely small fraction 
of the seeds are ordinarily carried to a substratum suitable for growth 
to maturity, enough seedlings usually become established to per- 
petuate the species. 

The tremendous numbers of animal larvae in the attaching stage 
are often an index of the critical nature of the attaching process. A 
graphic illustration of the intensity of the reaction for the species to 
attain a proper substratum was found in the study of the settling of 
barnacle larvae in experiments conducted in Biscayne Bay, Florida. 
Glass plates 20 x 25 cm in size were placed in the water each day, and 
the number of barnacle cyprids which attached to them was deter- 
mined. On one occasion the count showed that 3860 cyprids had 
settled on one glass plate during the previous 24 hours! When it is 



Reactions to the Substratum 61 

realized that the same intensity of attachment for these fouling or- 
ganisms extended throughout great areas of Biscayne Bay, the magni- 
tude of the reaction is appreciated. 

Reactions to the Substratum 

Other animals and plants, rather than relying on chance to reach a 
suitable substratum, actively seek it. Climbing plants often exhibit 
a tendency for the growing parts to keep in contact with solid surfaces. 
This differential growth in response to contact with a surface is termed 
stereotropism. Thus, the tendrils or stems of climbing vines twine 
around or adhere to objects with which they come into contact. In 
other species, such as euonymus, the vine simply presses against the 
solid surface without any special structure for attachment. Root tips, 
on the other hand, turn away from stones and other solid objects 
which they may encounter as they grow through the soil. This 
reaction might be thought of as a negative stereotropism. 

Certain animals exhibit a locomotory orientation to surfaces, a 
response known as thigmotaxis, by means of which they keep in con- 
tact with, or avoid, solid objects. This reaction can be observed in 
many insects, and among higher animals it is demonstrated by rats 
and house mice when they tend to keep in contact with a wall 
(Fraenkel and Gunn, 1940). 

Many worms and insects are stimulated to continue moving about 
until their bodies are in contact with surfaces of the environment as 
they would be when in burrows or under stones. In the laboratory 
insects will come to rest between the surfaces of glass slides, and 
this fact shows that the reaction is a positive response to touch 
rather than an avoidance of light. In nature, since there are no 
transparent, solid objects, the organism would also be hidden from 
view. If a caterpillar is placed on its back, it will immediately go 
through righting reactions until its feet are again in contact with the 
substratum. However, while the caterpillar is still on its back, if a 
leaf is placed in contact with its feet, the animal makes no further 
attempt to right itself. It will be perfectly content to remain upside 
down as long as its feet are firmly attached to a solid surface. 

A great many animals obviously exhibit the reverse reaction, 
avoiding solid objects, and ordinarily do so by the use of sight or 
touch. A most unusual method of avoiding collision in the dark is 
now known to be employed by bats (Galambos and Griffin, 1942). 
At night and in dark caves bats can fly rapidly about without running 
into each other or into jutting rocks or other objects. The success 



62 The Substratum 

with which these animals can navigate in darkness was clearly demon- 
strated on one occasion when a small group undertook the exploration 
of a cave in Cuba under the guidance of a local plantation supervisor. 
No sooner had the party passed through the entrance into the inky 
blackness of the cave than hundreds of bats sprang into the air and 
flew madly about. The air was thick with bats. At this point the 
wife of one member of the group demurred. Not wishing to have the 
lady of the party miss the trip, our guide offered her a dollar for each 
bat that struck her while passing through the cave. Not a single 
dollar had to be paid. Bats have been shown to emit supersonic 
vibrations which are reflected from obstacles and are detectible by the 
bats' ears. In this unique manner bats can avoid obstructions in their 
paths during flight through dark forests and tortuous caves. 

The Variety of Substrata 

Many different substrata exist in the natural community. Almost 
every object is a potential substratum. One often hears the state- 
ment, "Nature is relentless." This saying springs from *he fact that, as 
soon as any manmade object is abandoned, animals or plants attach 
to it, bore into it, or grow over it. Thus nature is relentless in re- 
ducing man's artifacts to natural conditions. 

The most common substrata are the many derivatives of rock; these 
will be considered shortly, but first let us take note of the great variety 
of substances which can serve as substrata. It is not even necessary 
for the substratum to be a hard surface. The surface film of water 
serves as the substratum for a category of organisms called the neuston. 
Many algae, certain higher plants such as the duck weed (Lemna) 
water striders, and "whirligig beetles" which inhabit ponds are sup- 
ported by the tension of surface film and use it as their regular sub- 
stratum. Flatworms and pulmonate snails are able to employ the 
underside of the water surface as a substratum. Under quiet condi- 
tions the observer can see these forms progressing steadily along the 
interface between the air and water. Mosquito larvae similarly 
can attach to the underside of the surface film (Fig. 3.1). The par- 
ticular ecological needs of the mosquito in this stage make it possible 
for man to exterminate the pest, locally at least, by applications of oil 
or poison to the surface of the water. When undisturbed by public 
health agents, however, the mosquito larva grows rapidly on food 
which has accumulated at the water surface. Particles sinking 
through the air accumulate on the water and other materials floating 
up from deeper layers come to rest under the surface. Hence the 



The Variety of Substrata 63 

air-water interface presents an excellent feeding ground. By pinching 
the surface film with its specially adapted mouth parts the larva ob- 
tains the food material and causes more distant particles to move 
nearer the area of its activity. In this way the larva is said to be 
able to gather food from a circle perhaps 30 cm in radius around its 
point of attachment. 



Anophelines 



Culicines 



Anopheles 

g i 

on water) 



Aedes 



Culex 




Water 
surface 




Fie. 3.1. Diagram of lite cycle of mosquito, showing the use of the water surface 

as a substratum in the early stages. The larvae use the surface film not only for 

support but also for a feeding ground. 

Another special substratum of interest is wood. In the air environ- 
ment "dry rot" fungi, termites, and other organisms find this material 
a suitable substratum for their activities and for their nourishment. 
Far from human habitation the destruction of dead timber by these 
plants and animals merely aids in the reduction of organic materials 



64 



The Substratum 



to simpler substances. But when these species attack buildings, tele- 
graph poles, and railroad ties unchecked, they cause tremendous 
damage, often within a short time. A knowledge of their ecological 
relations then becomes a matter of vital economic concern. Wooden 
structures under water similarly serve as the substratum for other 
rotting fungi, for the "shipworm" Teredo, and for other forms, which 
are consequently also of great economic importance. It is a curious 
fact that, although the shipworms honeycomb a piece of wood, their 
tubes rarely run into one another and practically never break through 
the surface of the wood to the exterior. In some way the boring 
shipworm is able to detect when it is nearing the limit of its substratum 
and stops itself from running out into the open. 




Woods Hole Oceanographic Institution 

FIG. 3.2. Mussels and other fouling organisms forming a crust 17 to 30 cm thick 

and weighing up to 13 kg per 1000 cm 2 on the bottom of a bell buoy seen as it is 

being hoisted out of the water off Cape Cod, Massachusetts. 

The hulls of ships and the surfaces of underwater structures present 
specialized substrata in the marine environment that are used as points 
of attachment for many kinds of fouling organisms including notably 
algae, barnacles, mussels, and tubeworms. Even the smooth steel 
bottoms of modern vessels and navigation buoys are rapidly attacked 
by such plants and animals (Fig. 3.2). An understanding of the 



The Variety of Substrata 65 

ecology of fouling is necessary for success in the very practical prob- 
lem of preventing this attachment. In experiments with different 
types of materials with which to coat submerged objects including 
ship bottoms, it was found that no surface could be devised which 
was so smooth, so slippery, or so soft that a barnacle cyprid could not 
gain a foothold. Tests showed that fouling could be prevented only 
by covering the ship's hull with paint that emitted copper or other ions 
strongly toxic to the attaching stages, as illustrated in Fig. 3.3 (Woods 




C. M. Weiss, Woods Hole Oceanographic Institution 

FIG. 3.3. Bottom of boat showing reactions of fouling organisms to different anti- 
fouling paints. Fouling has been prevented on port side by emission of ions from 
cuprous oxide paint. On the starboard side fouling organisms have attached be- 
cause the emission from the metallic copper paint used on that side has been in- 
hibited by the coupling action of the galvanized iron patch seen at right of 

propeller. 

Hole Oceanographic Institution, 1950). Without an effective anti- 
fouling paint a battleship is said to require 30 per cent more fuel to 
maintain cruising speed within 6 months after launching, and its top 
speed is seriously reduced. The research program on the ecological 
relations of fouling organisms and on poisoning methods undertaken in 
American marine laboratories during World War II was reported to 
have saved the United States Navy 10 per cent of its entire fuel bill. 
The substrata used by plants and animals are not restricted to in- 



66 The Substratum 

animate objects. The surfaces of other organisms are also susceptible 
to invasion by attaching organisms. Plants which grow on the ex- 
ternal surfaces of other organisms but do not obtain nourishment 
from them are called epiphytes. Similarly, animals which gain attach- 
ment or shelter without using the tissues of the host organism as a 
source of food are termed epizoans. If the attaching forms obtain 
nourishment from the host, they are external parasites. The bro- 
meliads and orchids perched on the branches of forest trees in the 
tropics are typical epiphytes. The barnacles on the backs of whales 
are examples of epizoans, although they are often erroneously referred 
to as parasites. 

When the substratum of an organism is another animal or plant, 
new relations appear. The organism may find that its substratum 
moves around, grows, or is destroyed. In such instances the inter- 
dependencies between the epiphyte or epizoan and its host become 
extremely complex. For example, the distribution of sessile rotifers 
attached to the water plant Utricularia depends upon the relative rates 
of the migration of the rotifers and the elongation of the stems of the 
plant (Edmondson, 1946). When the growth of the plant is rela- 
tively slow, the rotifer population tends to become concentrated. 
But, in the reverse situation, the rotifers become spread out with cor- 
responding changes in the age distribution of the population. 
Epizoans on the gulf weed Sargassum are similarly subject to the 
vicissitudes of a living substratum. The floating gulf weed elongates 
at its distal end and dies and breaks off at its basal end. A slow- 
growing hydroid attached to the Sargassum might not be able to 
maintain itself if its substratum grew out from under it. 

ROCK, SAND, AND MUD IN AQUATIC ENVIRONMENT 

Influence of the Aquatic Substrata 

Although many different materials can serve as substrata in the 
water environment, by far the most common are rock and its deriva- 
tives. Whether the substratum consists of smooth rock, loose stones, 
sand, or mud has a profound effect on the distribution of aquatic or- 
ganisms and on the regulation of their growth. Different textures, 
various degrees of stability of the material, and a great variation in 
the nutrient content have an important selective action. 

In shallow water the difference in fauna and flora on a rocky bot- 
tom, on a sandy beach, or in soft mud can easily be studied by anyone 
visiting the shore. On the sea coast, for example, a rock substratum 



Influence of the Aquatic Substrata 67 

will characteristically support a rich growth of brown, green, and red 
algae attached by "holdfasts*' and a wide variety of snails, mussels, 
sea anemones, starfish, and many other invertebrates secured by suck- 
ing or cementing devices. Some of these animals indirectly derive 
nourishment as well as attachment from the rock substratum. The 
snails, for example, scrape off and eat the slime that forms on all 
underwater surfaces as well as the bacteria and algae contained in the 
slime. In shifting sand or gravel few species except rapidly burrow- 
ing animals can maintain themselves; but on and in firm sand, espe- 
cially when mixed with mud, a distinctive and rich population of 
mollusks, worms, and crustaceans will be found, provided that other 
environmental factors are favorable. On a mud bottom where the 
water is quieter, rooted plants like the eel grass often grow in abun- 
dance, and sea cucumbers, brittle stars, sea urchins, and a different 
selection of worms are among the common inhabitants. However, if 
oxygen has been depleted and hydrogen sulphide is formed in the 
mud habitat, the benthic population will be greatly reduced. Further 




Photo mad. with Ewin g undersea camera by D. M. Owen, Wood, Hoi, Uc<ano g rap*<c insn.unon 

FIG. 3.4. Ripples on the sea floor at a depth of about 100 m on Georges Bank off 

Massachusetts. The numerous brittle stars and sand dollars visible in the 10 nr 

area shown are able to maintain themselves on the shifting surface. 



68 The Substratum 

examples of the control of bottom fauna and flora by the substratum 
in coastal water are given by Yonge (1949) and by Pratt (1953), and 
in fresh water by Krecker and Lancaster (1933) and by Wilson 
(1939). 

Farther offshore conditions cannot be so easily observed, but the 
modern quantitative dredge and the underwater camera have shown 
that the same selective action is being exerted by the nature of the 
bottom material. Samples dredged from mud and from coarse shell- 
gravel at locations only a few miles apart in the English Channel were 
shown by Wilson ( 1951 ) to contain strikingly different animal types. 
Such differences influence fish populations feeding upon these ben- 
thonic species. Studies made with the underwater camera have the 
advantage that they reveal the nature of the bottom material and the 
organisms living on it in their undisturbed condition. The dimen- 
sions of bottom features such as ripples, and the spatial distribution of 
the inhabitants may then be examined quantitatively (Fig. 3.4). In 
the abyssobenthic zone of the ocean the bottom material usually con- 
sists of a soft mud, and here only those animals with long legs, broad 
bases, or other special adaptation can move about without being 
smothered. At a depth of 1% miles the candid underwater camera 
caught a 60-cm "sea cucumber" as it cruised across the muddy bottom 




Fie. 3.5. A holothurian ("sea cucumber") moving over the mud at a depth of 

2600 m ( 1]X> miles ) on the floor of the open ocean off the coast of New York. 

Note the imprint of the double row of tube feet. 



Build-Up of the Substratum 69 

(Fig. 3.5). These holothurians, and other types of bottom animals, 
pass the mud through their intestines and extract the organic matter 
from it. For such forms the nutritive value of the substratum may 
have an even greater ecological importance than its physical nature. 

Breakdown of the Substratum 

We have seen the various ways in which the nature of the sub- 
stratum in the aquatic environment limits the growth and distribution 
of organisms living on it or in it. Turning the ecological picture the 
other way around, we find that the presence of organisms often has a 
profound influence on the substratum. Even solid rock can be bored 
into and broken down by animals living on it. In the intertidal zone 
of the Oregon coast sea urchins have carved craters for themselves in 
the sandstone. So abundant are the sea urchins and their craters that 
in many places the shelving rocks present a honeycombed effect. 
Even more remarkable is the ability of bivalve molluscs of the family 
Pholadidae to drill into gneissic rock and into concrete. Some of the 
pillars of the causeway to Key West have been pockmarked by these 
animals. Empty shells which form the major portion of the sub- 
stratum in some regions are broken down by the sponge Cliona. The 
basal part of this sponge produces an acid secretion that hastens the 
disintegration of the shells. In some situations the complete destruc- 
tion of an organism's substratum by its own activity has led to its 
undoing, for without a suitable substratum it is eliminated from the 
habitat. 

Build-Up of the Substratum 

In addition to the breaking down of solid materials many aquatic 
organisms play an important part in building up their substratum. 
Calcareous algae and many types of coral animals cause calcium 
carbonate to be deposited in and around their tissues. As a result 
limestone formations of various sorts are brought into being. Anyone 
who has visited the tropical ocean is well aware of the vast extent of 
reef -building activity (Fig. 7.12). In fresh water certain organisms 
similarly cause the precipitation of calcium carbonate. The marl 
deposits that are formed in this way often come to be the chief com- 
ponent of the substratum in ponds and lakes. 

In some situations the new substratum produced by the activity of 
living agents consists essentially of the surfaces of the organisms 
themselves. Occasionally a few mussels become attached to individ- 



70 



The Substratum 



ual stones scattered over a clam flat of fine sand or mud. During the 
next season young mussels may attach to the shells of the old mussels. 
Subsequent "sets" gain foothold on the surfaces of the second genera- 
tion until gradually a mat of mussels spreads across the area that had 
previously been covered by soft material quite unsuitable for the 
growth of a mussel population. Mussel mats of this sort have some- 
times been a serious economic concern because they have smothered 
valuable populations of clams beneath. 

In other situations the action of the living part of the ecological 
system is that of binding together loose particles of the bottom material 
so that a firm substratum is provided in place of the shifting sand or 
mud previously existing in the area. The eel grass that was formerly 
prevalent along our Atlantic coast played this important role in many 




Murray and Hjort, 1911, copyright Macmillan & Co. 

FIG. 3.6. A washed sample of pteropod ooze from the Indian Ocean showing the 

conical and angular shells of pteropods, the rounded tests of Foraminifera, and the 

remains of other types of plankton, enlarged about 10 X . 



Influence of the Land Substrata 71 

bays and estuaries. After the eel grass was killed by a disease in the 
early 1930*s, large amounts of bottom material were washed away from 
many sections of the shore. As the unprotected sand and mud was 
scoured out by the tides, a great many other plants and animals that 
had been living in the area were destroyed. Eventually the com- 
plexion of the whole ecological community became altered ( Stauffer, 
1937). 

Another way in which organisms can modify the substratum is 
through contributing their own remains. The sand on the famous 
Pink Beach in Bermuda is composed chiefly of coral fragments. In 
the deep sea some of the bottom oozes consist mainly of the skeletons 
of planktonic organisms ( Sverdrup, Johnson, and Fleming, 1942, Ch. 
20). Globigerina ooze formed by the accumulation of the shells of a 
genus of Foraminifera and pteropod ooze similarly composed of the 
shells of gastropods are examples of calcareous oozes (Fig. 3.6). Two 
important types of siliceous deposits are formed by the accumulation 
of the hard parts of radiolarians and diatoms, respectively. Radio- 
larian ooze is found in certain tropical waters, and diatom ooze is 
limited to colder seas. 

Aquatic organisms also act reciprocally on their own substratum 
by adding organic material to it. In a peat bog the substratum is 
practically 100 per cent organic matter resulting from the accumula- 
tion of vegetable material (Welch, 1952, Ch. 16). In inland waters 
and in the sea, fragments of dead organisms reach the bottom as an 
organic detritus. Seaweeds and shore animals that have died and 
been broken up by the waves contribute abundantly to this material, 
but farther from shore the plant and animal plankton are largely re- 
sponsible for the particulate organic material in the water. This 
detritus tends to settle out where the current has been sufficiently 
reduced. The excreta of worms, lamellibranchs, and various Crus- 
tacea have been reported to form as much as 40 per cent of the fine 
material in the mud of the Clyde Estuary. All these organic sub- 
stances gradually become incorporated into the substratum and serve 
as sources of nutriment for the mud-eating benthic animals (Twen- 
hofel, 1939). 

ROCK, SAND, AND SOIL IN TERRESTRIAL ENVIRONMENT 
Influence of the Land Substrata 

In the land environment soil is by far the most important substratum, 
but rock and special materials such as plants and plant products also 



72 The Substratum 

serve as ecologically significant substrata. As rock breaks down it 
produces areas of stony ground, gravel, and sand; and with the ad- 
mixture of organic matter soil is formed from the parent rock sub- 
stance. Each of these materials has its influence in controlling the 
growth and distribution of plants and animals. As in the aquatic en- 
vironment the land substrata provide purchase, shelter, attachment, 
and nourishment in varying degree according to circumstances. 

Land Surfaces and Animals. Physical differences in the land 
surfaces are correlated with special adaptations of animals inhabiting 
them. On rocky terrains and in regions with hard, open ground the 
running speed of animals is improved by the possession of small re- 
sistant feet usually with a reduced number of toes, as in the deer, ante- 
lope, and ostrich. Animals living in areas of soft sand, marsh, or 
snow are characterized by spread-out feet, like those of the camel, or 
of toes that present a large surface, like those of wading birds and the 
snow-shoe rabbit. Sand-dwelling lizards and insects similarly are 
enabled to move over loose sand by toes or legs widened by lateral 
scales or hairs. Other animals have feet especially adapted for climb- 
ing trees (squirrels), or for clinging to branches or leaves (tree frogs), 
or for dealing with other special substrata. 

In contrast to the species requiring rapid locomotion over the land 
surface is a large group of animals that burrow into the substratum. 
Many rodents, some birds and reptiles, and a great many insects, as 
well as other types of invertebrates, are built for effective digging in 
the ground. Certain species such as the mole, the earthworm, and 
many insects spend most of their lives underground. All these bur- 
rowing forms are limited to regions in which suitable soil conditions 
exist. For example, a tongue of soil running across the Florida pan- 
handle that is too dry for the burrowing of crayfish acts as an ecological 
barrier separating certain west Florida species of crayfish from species 
limited to areas farther to the east (Hobbs, 1942). 

The chemical composition of the soil substratum affects animals 
both directly and indirectly through their food. Land snails with 
calcareous shells are especially abundant on soils rich in lime, but 
the abundance of the shell-less slugs is not affected in this way. The 
shell of one species of Helix was found to weigh 35 per cent of the total 
weight of the snail in limestone regions but only about 20 per cent of 
the total weight in areas with soils poor in lime (Hesse, Allee, and 
Schmidt, 1951, Ch. 20). The bones of mammals likewise are heavier 
on limestone soils; this is especially true of deer that annually must 
grow new antlers sometimes weighing as much as 7 kg. It is no acci- 
dent that strongly built race horses are raised in the bluegrass pas- 



Soil and Its Action on Plants 73 

tures on the limestone soils of Kentucky. Many herbivorous mam- 
mals require an abundant supply of salt (NaCl) beeause sufficient 
sodium must be taken in to maintain a proper ionic balance with the 
large amount of potassium contained in their plant food. If an ade- 
quate amount of salt is not available in the halophytes of salt meadows, 
ruminants are forced to travel to "salt licks," or are excluded from the 
region entirely. 

The amount of organic matter in the soil is of vital concern to 
earthworms and other small invertebrates that use this material as a 
source of food. A great number and variety of small insects and 
spiders, of still smaller nematodes, and of microscopic Protozoa, which 
live permanently in the soil, also depend upon this organic matter 
for their nutrition. Thus the microfauna also is controlled by the 
chemical qualities of the soil, as well as by its compactness, dryness, 
and other physical characteristics. 

Soil and Its Action on Plants. An adequate discussion of the 
nature of the soil, its changes in time and space, and its influence on 
plants, and indirectly on animals through its effect on vegetation, 
would require a whole book in itself. Nothing more than an intro- 
duction to the subject can be given here. For a more extensive treat- 
ment of soil itself the reader is referred to Lyon, Buckman, and Brady 
(1952), Kellogg (1941), and the Yearbooks of Agriculture issued by 
the U. S. Department of Agriculture. Excellent chapters on the 
ecological relations of soils in relation to plants are to be found in 
Costing (1948) and Daubenmire (1947). 

Besides its ecological importance as a substratum, soil has im- 
measurably great economic importance. Fortunately we are rapidly 
becoming aware of the critical value of the productive capacity of 
soil as a support for civilization. Wolf anger ( 1950 ) has stated that 
"the soil of a nation is its most valuable material heritage." The cru- 
cial need for immediate soil conservation in almost all countries of the 
world has been ably pointed out by Osborn (1948), Vogt (1948), 
and others. A thorough understanding of the ecological relationships 
involved is essential for the intelligent use of our existing soils, for the 
prevention of further soil loss and degradation, and for the restoration 
of the fertility of worn-out soils. 

Soil represents an extremely complex matrix consisting of minerals 
derived from the parent rock of the area, organic matter of local origin, 
and substances carried in by various agents. The physical nature of 
a soil depends first of all upon its texture and structure. Texture is 
determined by the size of the constituent particles, and structure is de- 
pendent upon the aggregation of these particles in the undisturbed 



74 The Substratum 

soil into grains, clumps, and flakes. Soil particles are classified ac- 
cording to size in the accompanying table. 

Sand 1 . 00-0 . 05 mm in diameter 

Silt 0.05-0.002 

Clay < 0.002 

In a good loam all three of the categories are well represented. 
The type of structure into which the particles are arranged affects 
profoundly the porosity of the soil. It also controls the amount of 
surface which is presented on the one hand to the air and water 
moving through the soil, and on the other to the hairs of roots grow- 
ing in the soil. The relative proportions of the soil constituents and 
of the air and water present are extremely variable. In an average 
good soil about half the volume is commonly represented by pore 
space of which half may be occupied by air and half by water. The 
solid material of such a soil may consist of 95 per cent mineral par- 
ticles and 5 per cent organic matter. In tropical soils, however, 
organic matter may be less than 1 per cent, and in peaty soil it may 
approach 100 per cent of the dry material. In addition to differences 
in texture and structure, soils vary physically in the type of layering 
that they develop as they mature under biological and climatic in- 
fluences as will be discussed in the next section. 

The chemical nature of soils is even more diverse and variable. 
Upon the disintegration of the parent rocks the whole spectrum of 
minerals present becomes available for incorporation into the soil. 
Added to these are a wide variety of organic substances derived from 
animals and plants and other materials introduced from the air and 
ground water. Further chemical changes take place within the soil 
as climatic and biological agents work on it. As a consequence, soils 
and soil water differ widely in chemical composition, organic content, 
and total salinity, as well as in degree of acidity, oxidation-reduction 
potential, and other physicochemical characteristics. Some of these 
features of the soil are intimately interrelated with the physical char- 
acteristics. For example, the smallest particles involved in the texture 
of a soil are colloids, and their behavior and reactions are also in- 
volved in the chemistry and physical chemistry of the soil. The abun- 
dance and type of the colloids present affect the amount of water 
retained by the soil and its availability to plants. At the same time 
the colloids influence the chemical composition of the soil water. 

In the present section we are concerned with soil primarily in its 
physical nature as a substratum for land organisms. The foregoing 
brief sketch of soil has indicated the extremely complicated nature 



Soil and Its Action on Plants 75 

of this substratum and the degree to which its physical characteristics 
are bound up with its chemical and its physicochemical features. 
Certain of the latter will be further discussed in relation to other 
ecological factors considered in later chapters. 

When gross differences in the land substrata exist, factors limiting 
plant growth can frequently be distinguished. On a solid rock sub- 
stratum lichens and certain mosses are characteristically the only 
plants that can survive. In situations with a coarse, shifting sub- 
stratum, like a sand dune or a gravel slide, the vegetation is limited to 
specially adapted forms. Dune grasses with their network of hori- 
zontal rhizomes, and certain other plants, such as Paronychia, with 
strong and extensive root systems are among the few plants that can 
maintain a foothold. When the soil is very hard owing to a high silt 
and clay content or to the development of a hardpan, the roots of many 
species cannot penetrate. At the other extreme the presence of very 
soft soils prevents the establishment of plants that require firm anchor- 
age. Many evidences of this relation were seen in New England 
after the hurricane of 1938 when whole groves of trees on loose soil 
were uprooted but neighboring groups of the same species on hard 
ground remained standing after the storm. 

In other situations a gross difference in such factors as the chemical 
composition, moisture, or temperature of the soil may be distinguish- 
able as the prime influence controlling the vegetation, and examples 
of these will be considered later in the appropriate chapters. Too 
oft^n for the peace of mind of the ecologist, however, very compli- 
cated or subtle differences occur in the soil, and frequently two or 
more interdependent factors appear to act mutually in limiting plant 
growth. More detailed treatments of the ecology of soils, such as 
those referred to earlier in this section, should be consulted for a 
further discussion and examples of situations of this type. In some 
habitats investigators disagree not only as to which of the soil influ- 
ences is critical in determining the composition of the vegetation but 
also even as to whether the climatic factors are not more important 
than the edaphic ( soil ) factors. 

These ecological relations in the soil also frequently illustrate the 
principle of partial equivalence: an increase in one factor may some- 
times partially make up for a deficiency in another factor (Allee et al, 
1949, Ch. 16). The lack of moisture in a sandy soil may be compen- 
sated for to some extent by a greater rainfall in certain localities, or, 
conversely, plant species for which a given region is generally too 
hui&d may find the effective moisture conditions sufficiently reduced 
in a local area with a sandy substratum. 



76 The Substratum 



Action of Organisms on Soil 

Having seen the many ways in which the nature of the land sub- 
stratum may influence the lives of organisms, we now may inquire to 
what extent the action is reversed. The fact is soon revealed that 
animals and plants play a very important part in modifying their sub- 
stratum on land just as they do in water. This activity on the part 
of terrestrial organisms is particularly striking in relation to the forma- 
tion and development of soil. It has been truly said that if it were not 
for organisms there would be no soil at least none of biological im- 
portance. The soil is an outstanding example of the result of the 
organism and the environment acting as a reciprocating system. 

Abundance of Organisms in Soil. The great abundance of organ- 
isms which live wholly within the soil and the far-reaching extent of 
the underground parts of organisms are not always appreciated. 
Anyone who spades up a garden or transplants a shrub should be 
impressed with the number of roots and the bulk of the root systems 
of even small plants. The roots of the typical plant are so finely 
divided and subdivided into rootlets and root hairs that a tremendous 
surface is provided for the exchange between the organism and its 
surroundings (cf Weaver, 1947). Roots are frequently sufficiently 
abundant to produce a continuous mat or network extending several 
feet into the soil. The root system of a maize plant may extend over 
1 m laterally and 2% m deep. The roots of 17-year old apple trees 
were found to have occupied all the soil between rows 10 m apart 
and to have grown to a depth of 10% m (Weaver and Clements, 
1938). 

In the animal kingdom the number of species that burrow through 
the soil and thus influence it is also very large. Burrowing rodents 
and moles of one kind or another exist almost everywhere, and they 
are much more abundant than is generally realized. Although in 
some instances the actual number of the larger forms may not be 
impressive, their digging activities may be remarkably extensive. 
Prairie-dog burrows more than 4 m deep have been reported. In 
certain parts of California systematic trapping has shown that as 
many as 50 mice inhabit each hectare (2% acres) under normal con- 
ditions. Periodically, as we shall see later, the rodent population 
tends to increase greatly in numbers. Even when the population of 
mice and other burrowing forms is at low ebb, a considerable in- 
fluence on the soil may be produced in the course of a year. In addi- 
tion to a variety of mammals, many kinds of reptiles and amphibians 



Soil Formation 77 

as well as a few species of birds spend at least a part of their lives 
burrowing in the soil. 

Of smaller animal forms, the numbers present in the soil are much 
greater (Chapman, 1931, Ch. 18). Earthworms have been estimated 
at hundreds of thousands per hectare, and their burrows may extend to 
depths greater than 2 m. Insects, especially in the larval stages, are 
very numerous in the upper centimeters of the soil (Salt et ah, 1948). 
In some regions population densities of several million soil insects 
per hectare have been found, Spiders, tardigrades, millipedes, and 
isopods are also abundant inhabitants of the land substratum. An 
extensive study in Illinois showed that the invertebrates of the soil 
reached an average summer maximum of 3300 per sq m-or roughly 
one animal under every 3 sq cm of surface. Since many of these 
forms are short lived and populations succeed one another in the soil, 
the study indicated that at least one or two invertebrate animals had 
existed during the year under every square centimeter of soil surface. 
In mineral soils of Jutland nematodes have been found to range in 
number from 175,000 to 20,000,000 per square meter (Nielsen, 1949). 

The numbers of microorganisms in the soil are, of course, much 
greater and produce a profound effect on the substratum. Protozoa 
may exist in concentrations of hundreds of thousands per gram of soil, 
The mycelia of fungi penetrate the soil wherever suitable conditions 
are found, and bacteria are extremely abundant almost everywhere. 
In raw humus as many as 20,000 bacteria per gram are a common 
occurrence. In rich loam the bacteria population may rise to 50 or 
even 100 million cells per gram of soil (Waksman, 1932). All these 
denizens of the soil from sizable burrowing animals to the smallest 
microorganisms add their influence to that of the underground parts 
of plants in modifying the substratum. 

Soil Formation. Soil is formed by the combined action of several 
agents. First may be mentioned the process of fragmentation, the 
mechanical breakdown of rock material into smaller pieces. The 
process is carried forward partly by geological agents, including 
especially the freezing and thawing of moisture in the ground. The 
action of roots in splitting rocks is also important and represents the 
biological part of the process. No matter how small the rock par- 
ticles may be, however, plants cannot obtain nutriment from the ma- 
terial until the minerals are rendered soluble. The second step in 
soil formation, and one that goes on simultaneously, is termed cor- 
rosion and includes the chemical processes of oxidation, reduction, 
hydration, hydrolysis, carbonation, and others. These processes go 
forward and soil materials go into solution under the influence of rain 



78 The Substratum 

after it percolates through the ground as soil water. But the action of 
pure rain water would be extremely slow. Root secretions added to 
ground water, including notably carbonic acid, cause the rock mate- 
rials to go into solution much more rapidly. If you look closely at a 
rock covered by lichens, you can see the results of the combined action 
of mechanical breakdown and corrosion due to plant secretions. 

The third factor taking part in the process of soil formation is the 
addition of organic matter. Plants contribute their deciduous parts 
at regular intervals, and when each plant dies it adds its whole 
body to the soil. Since one thinks of plants as growing by withdraw- 
ing material from the soil, one might ask how any net gain would 
result from the death of the plant and the return of this material to 
the substratum. The answer is that the plant builds additional 
materials from the air and the ground water into its tissues. The 
carbohydrate synthesized by the plant is formed from carbon dioxide 
and water absorbed from the environment. Certain bacteria, such 
as Azotobacter and Clostridiwn, living freely in the soil, are able to 
take nitrogen from the air and to use it in their constructive growth. 
Other nitrogen-fixing bacteria live in nodules on the roots of legu- 
minous plants to which they pass on the nitrogenous compounds that 
they have manufactured. Thus, more is added to the soil by the 
activity of the vegetation than is removed, and, what is more im- 
portant, the new material is in a very different chemical form. The 
inorganic substances taken up from the soil and from the atmosphere 
are converted into complex organic compounds by the growth proc- 
esses of the plant. When the plant dies, these organic substances 
become incorporated into the soil and many of them decompose only 
slowly. 

Animals living on and in the soil add their excreta regularly and 
contribute their own bodies to the substratum when they die. Bur- 
rowing animals of all sorts mix into the soil the organic remains that 
have been added to the top of the ground. Rodents and many kinds 
of insects play an important role in this regard, as well as the pro- 
verbial earthworm. A visit to a deciduous forest where earthworms 
are abundant will provide an opportunity for seeing the effectiveness 
of this animal in tilling the soil. If you remove from the surface of 
the ground the leaves that accumulated in the last few months, you 
find few leaf remnants from previous years. All the older leaves 
have been eaten by the earthworms, and their faeces have largely been 
discharged at subsurface levels. When constructing new burrows, 
earthworms deposit their casts of soil from the deeper levels upon the 
surface. This action of the earthworms in mixing the upper layers in 



Humus and the Colloidal Complex 79 

the deciduous forest helps to produce an entirely different soil profile 
from that found under coniferous trees. Because of the lack of the 
earthworm population in the typical coniferous forest as well as the 
slower rate of decay the fallen needles tend to accumulate year after 
year as successive layers on the surface of the ground. 

We have seen that living agents aid in the breakdown of the parent 
rock material, take part in the vertical mixing and the horizontal dis- 
tribution of soil substances, and add organic matter to the soil. The 
by-products of animals and plants, and their own bodies when they 
die, are the only source of organic compounds for the soil. These 
organic substances provide necessary soil components, modify the 
soil into many different types, and make possible the growth of a 
varied fauna and flora that would not otherwise be able to exist. 
The organic matter contributed to the soil contains a larger amount 
of energy than the inorganic substances from which they were formed. 
Living organisms therefore provide both potential and kinetic energy 
as well as materials in helping to build the soil. 

Humus and the Colloidal Complex. Soils are far more than piles 
of material derived from rocks and biological sources, Soils have 
organization. Structure, layering, and other aspects of soil arrange- 
ment are influenced by the animals and plants present, and, recipro- 
cally, the activities of the soil organisms are frequently controlled by 
the organization of the soil material. One way in which animal life 
and vegetation affect soil organization is through their contribution 
to the colloidal complex. The partially decomposed organic matter 
added to the soil by living components is known as humus, and this 
material combines with the finest clay particles to form the colloidal 
complex. Often referred to as the "heart and soul" of the soil, the 
colloidal complex plays many essential roles in its dynamic activity 
(Waksman, 1936). In the first place the presence of colloids de- 
rived from humus and other sources influences the water-holding 
capacity of the soil and the rates at which air and ground water can 
circulate through it. Water tends to move too freely through sandy 
soil. The addition of humus to such soil tends to bind the grains to- 
gether, to reduce pore size, and to increase the amount of water held. 
A contrasting situation is found in soil that is too dense because of 
an excessive amount of clay. Increasing the amount of humus present 
has the effect here of separating the soil material into clumps, and 
thus allowing better aeration, increased percolation, and easier root 
penetration. In both instances the structure of the soil has been im- 
proved. 
The colloidal complex also acts as a source of plant nutrients and 



80 The Substratum 

as a particularly desirable type of storehouse since materials are re- 
leased from it only gradually. As will be discussed more fully in 
Chapter 8, the critical plant nutrients of the soil consist of such in- 
organic materials as nitrate, phosphate, potassium, and calcium, as 
well as of certain organic substances. These are derived in part from 
the humus itself and in part from the breakdown of the mineral com- 
ponents of the soil. Many of the nutrient materials are held by the 
colloidal complex in loose chemical combination or physical adsorp- 
tion on the surfaces of particles. The reactions involved in the de- 
composition of soil components and in the association of nutrient mate- 
rials with colloids are extremely complicated, and the reader should 
turn to a treatment such as Lyon, Buckman, and Brady ( 1952 ) for a 
further discussion. The general point emphasized here is that the 
colloidal complex fulfills the important function of providing for the 
slow release of nutrient materials in such a way that they can be 
absorbed by plant roots as needed. 

The difference between the gradual delivery of nitrogen from the 
supply in the organic matter and the rapid exhaustion of soluble nitro- 
gen that is freely mobile in the soil has no doubt been observed by the 
reader for his own lawn or garden. When nitrogen fertilizers are 
added to the soil, they tend to be rapidly dissolved. In this condition 
the nutrient salts may be quickly leached away by rain and ground 
water, or they may produce a "flash" growth of the plants present. 
More desirable for the growth- of cultivated plants, as well as for vege- 
tation in general, is the slow availability of nitrogen from the organic 
colloidal complex and from the decomposing organic matter, although 
nitrogen fertilizers are often beneficial if used properly. 

The Soil Profile. A broader aspect of the organization of the soil 
and another on which animals and plants exert a profound influence 
is the soil profile. If you look closely at a fresh vertical section 
through the soil, as exposed in an excavation or a road cut, you will 
see a succession of layers, or horizons as they are called, that together 
form the soil profile (Fig. 3.7). This arrangement of the soil mate- 
rial in layers is the result of the action of the living components and 
the climatic influences of the region on the original parent material. 
In some regions this master organization of the land substratum has 
been in the process of formation for thousands of years. The nature 
of the soil profile is of crucial concern in respect both to the natural 
vegetation and to commercial crops. We should accordingly think 
twice before allowing agricultural procedures that may permanently 
destroy the established layering of the soil. 

The depth and composition of each horizon of the soil profile differ 



The Soil Profile 



81 




Photo U. S. Soil Conservation Service 

FIG. 3.7. Vertical cut through fine sandy loam supporting blue-stem grass at Red 
Plains Experiment Station, Guthrie, Oklahoma. 

A Horizon, 0-1.0 ft (0-30 cm): grayish brown fine sandy loam. 
B Horizon, 1.0-2.5 ft (30-75 cm): reddish sandy clay. 
C Horizon 2.5-3.2 ft ( 75-95 cm ) : weathered parent material. 
D Horizon below 3.2 ft: laminated sandstone and shale. 



greatly from place to place and change with time in the same region 
as the soil matures. A varying number of subdivisions of each hori- 
zon are recognized in different situations with the result that a com- 
plete analysis of the soil profile becomes very complex, In many 
soils some of the subdivisions are entirely unrepresented, and in other 
soils strata of considerable thickness may be uniformly mixed, The 
general nature of the principal horizons and subdivisions are shown 
diagrammatically in Fig. 3.8. 



82 



The Substratum 



The process involved in producing and maintaining a typical soil 
profile are indicated schematically in Fig. 3.9. Here is shown one 



Horizons 
I r i 



True soil 



Litter of undecomposed organic debris 
Organic debris partly decomposed or matted 

High content of organic matter 
mixed with minerals 

Zone of maximum leaching 
Transitional 



Zone of concentration of transported 
fine particles 



Weathered or unconsolidated parent material 



Rock or unmodified parent material 



FIG. 3.8, The soil profile with its principal subdivisions shown by a vertical sec- 
tion through the soil. 

member of a stand of trees that has grown in the area for a long time 
and that has produced in the soil the characteristic layering. Per- 
haps the most important of the climatic agents is the moisture factor, 
and this involves both rainfall and evaporation. Where the land is 
sloping a portion of the rain runs off over the surface, sometimes wash- 
ing away organic materials or even some of the soil itself. The rain 
that enters the ground percolates through the pore spaces carrying 
fine particles and dissolved salts with it. In some places the ground 
water drains completely through the soil into the strata beneath, with 
the result that the dissolved materials may be lost from the soil. 
With excessive rainfall valuable constituents may thus be leached 
from the upper horizons. Other portions of the ground water enter 
the roots of the vegetation and are carried upward again. Water 
vapor moves through the pore spaces and evaporates from the sur- 




s 
1 






2 

-H ^ 
rt S 



o 



CO 

o 



83 



84 The Substratum 

face. The circulation of soil air provides for the transport of oxygen 
and carbon dioxide necessary for or resulting from the metabolism 
of the plant roots, soil animals, and microorganisms. 

The diagram also indicates the photosynthesis of the vegetation and 
the transfer of organic substances formed by the foliage to the roots 
deep in the soil. Organic compounds in great variety are elaborated 
by the growth of associated plants and animals. As described earlier, 
the Deciduous parts of plants, the excrement of animals, and finally 
the bodies of all these organisms, when they die, are added to the 
soil. These organic remains accumulate on the surface, or are carried 
to deeper levels by the soil fauna, where they begin the slow process 
of decomposition. 

This interplay of biological and climatic agents working on the 
parent material results in the production of the soil profile as indi- 
cated in the diagram. The A horizon is the scene of the major 
biological activity, and here organic matter chiefly accumulates. 
Rain entering this layer and percolating through tends to leach out 
the soluble salts. The A horizon is usually darker in color and lighter 
in texture than the B horizon below it, from which it is often sharply 
distinguished. The B horizon is characterized by less intense biologi- 
cal activity. Fine particles tend to accumulate here and mineral 
salts are often concentrated in this layer. The material of the B 
horizon is usually bright in color and densely compact. The B hori- 
zon tends to grade rather indistinguishably into the C horizon below 
it where there is little or no biological activity. As the soil matures, 
the A and B horizons tend to deepen until equilibrium conditions are 
reached. 

The exact nature of the mature soil profile depends upon the par- 
ticular balance reached between the climatic factors, the biological 
agents, the contour of the land, and the type of the parent rock in 
the given area. The many possible variations and complications 
should be borne in mind in a consideration of the simplified ex- 
ample outlined above. It should be amply clear, however, that the 
condition of the soil is a result of the elaborate interaction between 
organisms and their environment. The dynamic viewpoint of the 
modern ecologist is, therefore, especially appropriate in the further 
investigation of the problems of the soil that have been merely touched 
upon here. As one further illustration let us consider briefly the in- 
teraction between the vegetation and the substratum that underlies 
the differentiation of two great categories of soils. 

Soil-Group Divisions. Soils are classified into groups, and in the 
United States soil groups fall into two great divisions: the pedocals 



Soil-Group Divisions 



85 



and the pedalfers, in the terminology used by Wolf anger (1950) and 
shown in Fig. 3.10 or the aridic and the humid soils, in the some- 
what different classification used by Lyon, Buckman, and Brady 
(1952). The pedocal division is composed of incompletely leached 
soils found characteristically in the arid Great Plains of the West. 
The slight rainfall of these regions does not saturate the soil to a depth 
sufficient to reach the water table deep in the ground, and lime tends 
to be deposited in the B horizon. When evaporation from the soil 
begins, the ground water is drawn upward again toward the surface, 
carrying some of the solubles with it. The water in this kind of soil 
has been said to be "hung from the top, like Monday's wash!" The 
roots of the grasses and shrubs that are characteristic of the pedocal 
soils absorb the water and the contained salts and also carry them to- 
ward the surface. Grasses particularly tend to absorb a considerable 
amount of calcium and to restore this material to the upper layers. 
This process is aided by the low growth habit of these plants and the 
great development of rhizomes and other structures near the surface 



PEDALFERS 




PEDOCALS 
I BLACKERTHS 
] DARK-BROWNERTHS 
| BROWNERTHS 
1 GRAYERTHS 



[i;,'i,'i;ij| (JNDIFFERENTIATED HIGHLANDS 
| ,] SANDHILLS 



PEDALFERS 

GRAY BROWNERTHS 

RED-AND-YELLOWERTHS 

PRAIRYERTHS 

PODZOLS 



FIG. 3.10. The soil groups of the United States and their division into Pedocals 
and Pedalfers. (Modified from Wolf anger, 1950, in Conservation of Natural Re- 
sources, reprinted with permission of John Wiley and Sons. ) 



86 The Substratum 

of the ground. For these various reasons calcium tends to accumulate 
in the upper layers of the soil, and from this fact the name "ped-o-cal" 
is derived. The retention of calcium carbonate and magnesium car- 
bonate helps to prevent pedocals from becoming acid. 

An outstanding example of the interaction of climatic, edaphic, and 
biological agents in the development of a pedocal soil is furnished by 
the chernozems, or blackerths, that occur from the Dakotas south- 
ward. The climate is less arid than it is farther west, and a good 
grass cover is typically present. The action of the ground water and 
of the vegetation in retaining salts and other nutrient materials in the 
upper layers and in maintaining a neutral or slightly alkaline condi- 
tion is indicated schematically in Fig. 3.11. Organic matter tends to 
accumulate in the A horizon, resulting in the development of the rich, 
dark soil condition familiar in the region. Chernozems are conse- 
quently especially valuable for grazing or for farming because when 
left undisturbed, or properly managed, the ecological processes pres- 
ent tend to perpetuate soil fertility. 

The pedalfer division consists of soil groups found principally in 
the more humid regions of the eastern half of the United States. 
Here the rainfall is heavier with usually more than 75 cm falling per 
year. The solubles of the soil tend to be carried beyond the reach of 
the roots by the large amount of water percolating through the upper 
horizons. Much of this water filtering through the soil reaches the 
water table and drains off valuable nutrients. The roots of the typical 
forest vegetation growing on such soils extract salts but do not remove 
relatively as much calcium as do the grasses. The annual leaf-fall is 
on the surface, and some of the organic matter resulting from decay 
is carried away by the run-off, particularly in hilly regions. This loss 
of nutrient materials is in marked contrast to the situation with the 
pedocals and the grass vegetation of the prairies. 

In the north-central and northeastern parts of the United States 
the increased rainfall and lower temperatures tend to accentuate the 
processes just described. Soils of the podzol group, which occur in 
these regions, represent an extreme development of the conditions 
found in the pedalfer division (Fig. 3.12). In the podzols the 
solubles are rapidly lost by leaching. The A horizon becomes acid 
because of the accumulation of leaves, needles, and other organic 
debris on and in the surface strata. Calcium is dissolved from the 
upper layers and is often almost completely extracted from the soil. 
Aluminum and iron are similarly carried down but tend to be precipi- 
tated again as silicates in the less acid B horizon, and may cause the 
formation of hard-pan at this level. The tendency toward relative 



Soil-Group Divisions 87 

accumulation of aluminum and iron generally in this soil-group divi- 
sion provides the derivation for the term "ped-al-fer." 

In the region where the pedalfers exist there is generally sufficient 
rainfall for agriculture, but the soil tends to lose its nutrients. In the 
extreme podzol type the interaction of the climate and vegetation is 
such that solubles are rapidly carried away and the soil becomes 
progressively poorer and more acid, whereas in the chernozem soil 
fertility tends to be perpetuated. The contrast between the cherno- 



COVER OF 

GRASSES 

AND HERBS 

Organic matter 
mixed with 
mineral soil 

TRUE SOIL 

(Neutral to < 

slightly 

alkaline) 



Lime (CaCo 3 ) 
accumulation 



DRY SOIL 
MATERIAL 



Soluble 
calcium magnesium 
nitrogen silica 
potassium iron 
aluminum etc. 




1.5m- 



FIG. 3.11. Schematic vertical section through a soil of the chernozem group as 

representative of the pedocal soil-group division, The subdivisions of the soil 

profile are indicated in relation to the vegetation and the movement of soil water. 

The depth scale is representative, but varies greatly. 



88 



The Substratum 



zem soil as a representative of the pedocal and the podzol as a rep- 
resentative of the pedalfer illustrates the effectiveness with which the 
activity of organisms coupled with differences in climate can modify 
the substratum. The soil in its development and its organization is 



Organic material 
aluminum iron 
ited 



Calcium 

nitrogen 
potassium etc. 
eached from soil 




FOREST COVER 



Raw litter 

Decomposing A <*> l Br wn L 
litter 

Siliceous 
mineral soil 

TRUE 
(Acid 
reaction) 

Compacted 
zone 



WEATHERED 

ROCK 

Ground water level 
(fluctuates) 
2m 



W OUftV 7* 

FIG. 3.12. Schematic vertical section through a forest soil of the podzol group as 

representative of the pedalfer soil-group division. The subdivisions of the soil 

profile are indicated in relation to the vegetation and the movement of soil water. 

The depth scale is representative, but varies greatly. 



Soil-Group Divisions 89 

thus seen to be an outstanding example of a system formed by the 
interaction of organism and environment. 

The review of the substratum as an ecological factor given in this 
chapter has revealed the great variety of surfaces and solid materials 
on and in which animals and plants live. The manner in which the 
nature of the substratum controls the distribution and the growth of 
different species has been indicated. We have also stressed the fact 
that the activities of living organisms and the material of their bodies 
after death may profoundly alter the substratum in both the aquatic 
and the terrestrial environments. Sometimes this process itself brings 
about the further progressive changes in the fauna and flora known as 
ecological succession, considered in Chapter 12. Most of all, our 
present discussion focuses attention upon the reciprocally dependent 
relation existing between the living community and the substratum. 
The nature of the mud on the bottom of a pond or of the soil on the 
surface of the land is partly the result of the biological influences and 
is at the same time partly responsible for their existence. 

The physical conditions of the water or the land originally allow 
certain plants and animals to exist in a given area. These living 
agents may then modify the substratum, whereupon the substratum 
may be further affected by the "climatic" factors, perhaps resulting 
in additional changes in the fauna and flora. When we visit the area 
after these activities have gone on for a long time, and are still going 
on, it is not easy to distinguish cause and effect. Nevertheless, with 
a good grasp of the relations summarized in this chapter we are better 
equipped to determine the specific action of climatic, physiographic, 
and biological influences in bringing about the existing conditions. 



4 

Water 



Water plays several different roles in the ecological relations of plants 
and animals. In Chapter 2 we considered the physical characteristics 
of water in its mechanical action as a medium. In this chapter we 
shall discuss water as a substance taking part in the living complex, 
and in subsequent chapters we shall see how water may modify the 
action of other factors. 

As a material entering the organism water is important as a neces- 
sary and abundant constituent of protoplasm, and the plant or animal 
body as a whole generally contains a large percentage of water 
sometimes 90 per cent or more. Water is essential also as one mate- 
rial taking part in the photosynthetic reaction, through which energy 
becomes available either directly or indirectly to all living beings. 
Water is necessary as a solvent for food and as an agent for the chem- 
ical transformation of the materials within the body. Plants obtain 
mineral nutrients from the soil after they are in aqueous solution. 
Although most animals take in their food in solid form, this material 
must be dissolved before it can be absorbed by the blood and tissues. 
The intestines could be stuffed full of solid food and yet the animal 
would starve if no water were available for its digestion. Further- 
more, water serves as a vehicle for transport or circulation within the 
bodies of organisms. Minerals are carried up the stem of the plant 
by the transpiration stream. Water is the principal constituent of 
the circulatory, excretory, and reproductive fluids of animals, and it 
is necessary as a transfer agent at respiratory and olfactory surfaces. 
Water also acts as a regulator of temperature for plants and animals. 

For these essential purposes the proper concentration of water must 
be maintained inside the organism, and, at the same time, the transfer 
of water must be suitably regulated. If the organism could live 
hermetically sealed in a capsule, the retention of the necessary amount 
of water would be easy, but exchange of this material between the 

90 



Composition of Natural Waters 91 

organism and the outside world is necessary. The crucial need for a 
proper water balance is demonstrated by the consequences of even 
small water losses in some instances. During starvation, for example, 
man may lose as much as 40 per cent of his body weight including 
half of the proteins and nearly all the glycogen and fat without serious 
danger, but if 10 per cent of the water content of the human body 
is lost, serious disorders result. If as much as 20 per cent of the 
water is lost, death follows. 

The concentration of water divides the environment into aquatic 
and terrestrial habitats. At first sight, one might say that more than 
enough water exists in the aquatic environment and that water is a 
problem only on land. Closer scrutiny shows, however, that water 
is tending to enter or to leave the organism too fast in almost every 
situation. In certain terrestrial habitats the water supply may be 
excessive for many organisms, as in some tropical rain forests where 
the air is often 100 per cent saturated and the ground is completely 
permeated. A visitor to such a habitat sees moisture condensing on 
every surface and hears the steady dripping of water from the 
vegetation. 

In the ocean in spite of the thousands of cubic miles of sea water 
a scarcity of water would nevertheless exist for many plants and 
animals! For these organisms the concentration of water is too low 
relative to the abundance of salt for proper osmotic equilibrium. 
Fresh-water environments, viewed similarly in relation to osmotic 
balance, tend to contain too much water. This contrast between 
marine and inland aquatic situations is, in a sense, analogous to the 
difference between dry and humid climates on land. The mainte- 
nance of the proper water balance is thus a problem that must be 
considered in all habitats for all types of organisms. 

WATER PROBLEM IN THE AQUATIC ENVIRONMENT 

Since the water in natural environments contains a varying amount 
of dissolved materials and usually a different amount from the fluids 
of the organism, the resulting differences in osmotic pressure raise a 
problem in regard to water exchange. In order to determine the 
tendency of water to enter or to leave the organism we must know 
something of the composition of the surrounding medium. 

Composition of Natural Waters 

Representative values for the amounts of the more numerous ions 
found in sea water and in the water of ponds and lakes are shown in 



92 



Water 



Table 3. A larger amount of each of the common ions occurs in sea 
water than in typical fresh water. Hard fresh water contains more 
dissolved salts than soft fresh water, particularly with respect to the 
calcium and the carbonate ions. A comparison of the relative abun- 
dance of ions in each type of water may be made by recalculations in 
which sodium is given a value of 100 in each case ( Table 4 ) . In sea 
water chloride and sodium are the first and second most abundant 
ions, respectively, whereas in typical hard fresh water carbonate is 
the most abundant with calcium second. In typical soft fresh water 
the calcium and carbonate ions are relatively less concentrated than 
sodium and chloride. 

TABLE 3 

COMPOSITION OF SOME TYPICAL NATURAL WATERS 
(After Baldwin, 1948) 



Cl 



Total 
S0 4 CO 3 (g/liter) 



0.019 0.007 0.012 0.065 



Na K Ca Mg 

Soft fresh 

water 0.016 0.010 
Hard fresh 

water 0.021 0.016 0.065 0.014 0.041 0.025 0.119 0.30 
Sea water 10.7 0.39 0.42 1.31 19.3 2.69 0.07334.9 

TABLE 4 

RELATIVE ABUNDANCE OF IONS IN NATURAL WATERS 





Na 


K 


Ca 


Mg 


Cl 


S0 4 


C0 3 


Soft fresh 
















water 


100 





62.5 


3.3 


119.0 


43.8 


75.0 


Hard fresh 
















water 


100 


76.0 


310.0 


66.8 


195.0 


119.0 


567.0 


Sea water 


100 


3.6 


3.9 


12.1 


181.0 


20.9 


0.7 



Samples of sea water from different localities are surprisingly con- 
stant in respect to the relative abundance of the major constituents. 
So much is this the case that for many purposes it is not necessary to 
measure the concentration of more than one ion. The standard pro- 
cedure for determining the salinity of sea water is to find the amount 
of chloride ion present by titration and then to calculate the total 
salinity and the concentration of the other ions from the known ratios. 
The individual salts occurring in fresh water by contrast, vary widely 
often with far-reaching consequences that will be considered later. 

At the moment our concern is with the maintenance of the proper 
osmotic balance between the inside and the outside of the organism. 



Composition of Natural Waters 



93 



Since osmotic pressure is determined by the total concentration of 
molecules and ions in solution, values for the aquatic medium will 
vary according to the total salt content of the water body (Table 5). 
Dissolved materials in soft fresh water vary in amount from prac- 
tically zero to about 65 parts per million. Lakes and ponds with 
hard water exhibit a variety of higher values for salt content, but 
the value given in the table, 300 ppm, is representative. Salt lakes 
may contain exceptionally large amounts of dissolved substances. 

In contrast to the extreme variability in the salt content of inland 
waters, the open ocean tends to remain highly uniform in salinity 
over huge areas (Fig. 4.1), A fish could swim up the middle of the 
Atlantic Ocean, for example, from the Cape of Good Hope almost 



50 




50 60 



60 50 40* 



FIG. 4.1. The surface salinity of the Atlantic Ocean. 



94 Water 

to Iceland without encountering a salinity, lower than 35% or higher 
than 37%o (parts per thousand). Ocean areas with unusually high 
evaporation, like the Mediterranean Sea and the Red Sea, exhibit 
salinities that run up to 40%c. On the other hand, the salinity of the 
Baltic Sea is reduced to 8% or less due to the large inflow of fresh 
water. 

In river estuaries and in coastal areas receiving large amounts of 
run-off, salinity may vary over the whole range from nearly fresh 
water to full sea water. In smaller habitats, such as rock pools filled 
by ocean spray, the salinity may be typical of ordinary sea water on 
one occasion, may be greatly increased by evaporation a few days 
later, and may be reduced almost to zero by rainfall on another day. 
However, for the marine environment as a whole, such situations are 
definitely exceptional. The great length and breadth of the ocean 
maintains a nearly constant salinity of about 35 to 36% G . 

TABLE 5 

TOTAL SALT CONTENT OF SOME TYPICAL NATURAL WATERS 

Soft fresh water < 65 . ppm . 065 fa . 0065 % 

Hard fresh water 300.0 0.300 0.03 

Devils Lake, N. D. 11,000.0 11.0 1.1 

Great Salt Lake 170,000.0 170.0 17.0 

Baltic Sea 8.0 0.8 

Ocean water 35 . 3.5 

Red Sea 40.0 4.0 

TABLE 6 

TYPICAL VALUES FOR OSMOTIC PRESSURE EXPRESSED As FREEZING POINT 

DEPRESSION (=A) 

AWater AInvertebrates ATeleost Fishes 

Mediterranean 2.14 2.2 0.8-1.0 

Coastal Atlantic 1.79 1.8 0.7-0.8 

Fresh water . 03 0.1-0.8 0.5-0.6 

The osmotic pressure of a molar solution of a non-electrolyte is 22 
atmospheres or 1700 cm of mercury. The freezing point of such a 
solution is depressed to 1.84C, and this fact provides a convenient 
method for measuring osmotic pressure. In a solution of electrolytes 
with the same number of molecules the osmotic pressure would be 
higher because of the dissociation. Sea water is a 0.55 molar solu- 
tion, and, since the salts are dissociated, sea water has an osmotic 
pressure almost equal to that of a molar solution of a non-electrolyte. 



Methods of Meeting Osmotic Problems 95 

The salt content of the water in alkali soils may be even greater. 
Animals and plants living in sea water or in saline soils are therefore 
surrounded by liquids having a very high osmotic pressure. 

Methods of Meeting Osmotic Problem 

If the osmotic pressure inside the organism is the same as that out- 
side, relatively little difficulty is experienced in adjusting water 
equilibrium. If the osmotic pressure of the fluids of an animal or 
plant departs greatly from that of the outside medium, a problem re- 
sults in maintaining the proper water balance. If an animal moves 
into an area of very different salinity, a further adjustment will be 
required. 

Sample values for the depression of the freezing point (Table 6) 
reveal the fact that because of the higher salinity the osmotic pressure 
of Mediterranean water is higher than that of Atlantic water. The 
latter is very much higher than a typical value for fresh water. The 
osmotic pressure of the internal fluids of plants and of invertebrates is 
generally equal to, or higher than, the medium in which they are 
living. Among elasmobranch fishes the osmotic pressure is main- 
tained at different levels but is always higher than the surrounding 
water due to the retention of urea in the blood. 

The osmotic pressure of teleost fishes tends to remain within a 
relatively narrow range for both fresh- water and marine species. In 
fresh water the teleost is osmotically superior to its medium, whereas 
it is osmotically inferior in salt water (Fig. 4.2). In the sea a strong 
tendency exists for the fish to lose water from its tissues to the sur- 
roundings. A brief calculation will indicate the magnitude of the 
osmotic force with which such an animal must contend in order to 
maintain its water balance. If the osmotic pressure of the fish is 
represented by A = 0.7 and that of the surrounding medium 
by A = 1.8, the magnitude of the pressure corresponding to this 
difference of A 1.1 is: 

X 1700 = 1015 cm Hg 



1.84 

An osmotic pressure of 1015 cm Hg, or 13 atmospheres, is tending to 
extract water from the fish's tissues! For the teleost, therefore, and 
for other animals and plants with low internal osmotic pressures sea 
water has the effect of being physiologically dry. Fresh-water ani- 
mals and plants that migrate, or are carried, into the sea are indeed 
entering an arid climate. 



96 



Water 



The easiest way for an organism to deal with the osmotic pressure 
of its environment is to establish an internal osmotic concentration of 
the same magnitude. Even better, if the tissue fluids can be main- 
tained at a slightly higher pressure, then there will be a tendency for 
the needed water to enter the organism from the surrounding medium. 
This is the general method followed by plants and by invertebrate 
animals. 





A Water 

FIG. 4.2. Schematic diagram of osmotic pressure differences between elasmo- 
branch and teleost fishes and their fresh-water (left) and marine (right) media. 
The degree of shading indicates the relative values of osmotic pressure. ( Modi- 
fied from H. W. Smith, 1936). 



Most fresh-water and terrestrial plants can maintain an internal 
osmotic pressure ranging up to 2 atmospheres but they find saline 
habitats with a higher osmotic pressure physiologically too dry and 
uninhabitable. However, specially adapted plants, the halophytes, 



Limiting Effects of Salinity 97 

are able to tolerate higher salinities and to absorb water by virtue 
of the fact that the osmotic pressure of their tissue fluids is excep- 
tionally high. Halophytes sometimes exhibit internal osmotic pres- 
sures as great as 35 or even 40 atmospheres. Such plants inhabit the 
margins of the ocean and of salt lakes and can grow in saline soils 
with salt contents considerably greater than sea water. Although the 
exact relationship is not understood, it is of interest to note that 
halophytes, living in a physiologically dry habitat, exhibit succulence, 
heavy' cutin, and other xeromorphic characteristics familiar in the 
plants of the physically dry desert. 

In fresh water the tissue fluids of invertebrate animals have a con- 
siderable osmotic superiority because of the typically low salt content 
of ponds and lakes (Krogh, 1939). The chief problem here is a 
matter of getting rid of the excess water which enters through the 
membranes. In marine invertebrates the osmotic pressure of the body 
fluids is ordinarily only slightly, if any, higher than that of the sur- 
rounding sea water. When invertebrates living in estuaries move into 
fresher water, they must exert more osmotic work to eliminate the 
larger amount of water that tends to enter (Baldwin, 1948). The 
teleost fish must actively regulate water transfer both in the sea and 
in inland waters to prevent excessive loss of water in the former and 
excessive intake of water in the latter. A few animals can tolerate 
exceptionally high salinities as is exemplified by a fish that inhabits 
Japanese rock pools with salinity of 60%o and insect larvae that live in 
water of 42 to 62%o at Dry Tortugas Island (Pearse, 1950). 

Limiting Effects of Salinity 

The extent to which the various species can endure changes in 
osmotic pressure influences their activity and range. Organisms with 
a low tolerance for differences in salinity are known as stenohaline 
forms. Species that are limited exclusively to the open ocean or to 
fresh water are thus classed as stenohaline. Others may be limited 
to a narrow range of salinity of an intermediate value, but this is 
relatively rare. Plants and animals that are able to tolerate a wide 
range of salinities are termed euryhaline. Thus organisms inhabiting 
an estuary and fish that migrate back and forth from fresh to salt 
water are euryhaline. Such fish as the salmon and the eel are able 
to regulate their water balance in either a hypertonic or hypotonic 
medium. 

It should be noted that no sharp line or numerical limit distinguishes 



98 Water 

stenohaline and euryhaline forms these terms are entirely relative 
The terms also do not bring out the important difference in the effect 
of time of adaptation. Some species can withstand wide differences 
in salinity if they become adapted slowly, but are unable to tolerate 
rapidly changing salinity. Relatively few species can endure both a 
wide range and a rapid change in salinity. Interestingly enough, 
some euryhaline organisms may grow best in intermediate salinities, 
or even require them for a part of their life cycles, although our 
knowledge is scanty in this area, The division rate of certain species 
of phytoplankton has been shown in laboratory cultures to be twice 
as great at a salinity of about 20% than at salinities of about W%o 
or 30%o (Braarud, 1951). We also know that the American oyster 
can live in full sea water and can tolerate fresh water for short 
periods, but observations indicate that the larvae of this species will 
not settle at salinities above 32% nor below 5.6%<j. The optimum 
salinity for settlement in this estuarine animal is 16 to I8.6%c. Chang- 
ing salinities in estuaries may control the growth, reproduction, and 
distribution of more of the inhabitants than we now realize. 

Limits of distribution are sometimes sharply determined by salinity 
toleration. The vegetation around the margins of salt lakes and in 
the spray zone near the ocean shore exhibits a characteristic grada- 
tion of species from those most tolerant of salt to those least tolerant. 
In San Francisco Bay the slight difference in tolerance between two 
species of pile-boring mollusks was of great economic importance. 
In the upper regions of the Bay wharves and other wooden structures 
had been built without any special protection against shipwonns be- 
cause the local species of the genus Bankia was unable to grow in 
salinities below 10%o which frequently occurred in the area. In 1913 
another species of wood-boring mollusk, belonging to the genus 
Teredo, was introduced into San Francisco Bay, and this species 
could tolerate salinities as low as 6% . Spreading rapidly, Teredo 
attacked the wooden structures of the area and within a few years 
had caused destruction amounting to more than $25,000,000. 

Many interesting problems in relation to salinity tolerance remain 
unanswered. In most cases we do not know why one species can 
tolerate a rapid or extensive change in the salt content of its medium 
and another cannot. Why the euryhaline species have not come to 
dominate the open ocean as well as the coastal regions is unknown. 
Another question of interest is how oceanic birds and mammals can 
balance their water budgets without a supply of fresh water. Many 
of these forms eat marine invertebrates whose salt content is nearly 



Swamps and Temporary Pools 99 

as great as an equal volume of sea water. It may be that the water 
contained in the tissues of the food and the metabolic water are suffi- 
cient for the needs of marine birds and mammals ( Clarke and Bishop, 
1948), or these animals may have some special adaptation for excreting 
excess salts. 

AMPHIBIOUS SITUATIONS 

It is difficult enough for animals and plants to move from the sea to 
fresh water or vice versa, but, when aquatic organisms attempt to in- 
vade dry land, they are flopping out of the frying pan into the fire as 
far as the water problem is concerned. Nevertheless life on land 
does have certain advantages. Plants usually find more light, better 
anchorage, less abrasive action, and more concentrated nutrients. 
Animals may make use of the more abundant vegetation for food 
and shelter, the greater oxygen supply, and the possibility of more 
rapid movement. Against these advantages there is one signal dis- 
advantagethe scarcity of water. Success in colonizing dry land has 
been dependent on securing and retaining sufficient water. This 
could be easily arranged, perhaps, if the organism could seal itself up, 
but, as already stressed, surfaces must be left open for exchange with 
the external world. Of immediate importance is the problem of how 
to feed and to respire in air without drying up. Representatives of 
relatively few phyla have succeeded in becoming wholly independent 
of the aquatic habitat. However, some plants and animals have come 
part way out of the water, or come out for short periods, and we shall 
consider these first. 

Swamps and Temporary Pools 

Certain organisms have hit upon methods of using the advantages 
of both the air and the water environment. The emersed vegetation, 
for example, which grows in swamps or on the margins of lakes has 
its roots in the water and its upper portions in the air. In this position 
the roots find plenty of water available and extract nutrients from the 
mud, while the leaves are in the best position for receiving light from 
the sun and carbon dioxide from the atmosphere. However, many 
plants are not able to survive in a partially submerged condition be- 
cause of inadequate direct supply of oxygen for the roots. Plants 
that are adapted to existence entirely under water, or with their roots 
in water or in saturated soil, are known as hydrophytes. 



100 Water 

Amphibians are similarly able to utilize both air and water environ- 
ments. Many species can leave the water to forage on land, and, if 
their pools dry up, they can migrate to other bodies of water. Such 
animals are not limited by land barriers as most fish are. Yet am- 
phibians must re-enter the water or visit damp places at intervals to 
keep their skins moist, and most species need water for reproduction. 

Organisms living in swamps and pools must be able to deal with 
periods of drought when they occur. Physiological adaptations mak- 
ing this possible have been evolved sometimes without any accom- 
panying special morphological structures. Certain species of fish in 
India can live in wet grass for as long as 60 hours, and other animals 
burrow in the mud where they remain in a dormant condition during 
the dry period. Water mites have been shown to be able to live 
under debris for intervals up to 6 months after their pools have dried. 
The African lung fish, on the other hand, constructs a special mud 
cocoon and curls up inside for the duration of the dry spell. The 
fish secretes about its body an impervious sheath which prevents 
the loss of water from its tissues, and is thus enabled to survive drought 
for more than two years. Other specialized structures for tiding over 
dry periods are the spores and seeds of water plants, and the "resistant" 
eggs and cysts of various aquatic animals ranging from the Protozoa 
to the Crustacea. 

Another adaptation for meeting the water problem in temporary 
pools is that of speeding up development and taking quick advantage 
of water when it is available. In certain animals living in this type 
of habitat the larval stage is accelerated; in others parthenogenesis 
has been adopted. If a single female cladoceran such as the water 
flea, Daphnia, finds itself in a temporary pool, it does not have to hunt 
around for a male. It can go ahead and reproduce rapidly by par- 
thenogenesis. The young animals mature and continue reproducing 
parthenogenetically until in a relatively short period a large population 
has built up. Later, however, if the pool tends to dry up, the adverse 
conditions cause the appearance of males in the population. The 
females now produce a different type of egg, one which requires 
fertilization. Only one of these "winter" eggs is formed by each 
female and the brood pouch is modified into an ephippium (Fig. 
2-11). After the death and disintegration of the female, the ephip- 
pium remains as a resistant egg case within which the egg can survive 
prolonged periods of drought and even freezing. In this condition 
the egg may be blown into another pond or carried in dried mud by 
animals to another region. When water is again supplied to it, the 
egg hatches out and is able to start the cycle over again. 



Tidal Zone 101 



Tidal Zone 

Another situation that might be classified as amphibious is the 
tidal zone. Plants and animals living on the seashore between tide 
marks are subject to alternating droughts and floods twice each day. 
Organisms living in the center of the tidal zone must be able to be 
aquatic for about 6% hours when the tide is in, terrestrial for about 
6% hours when the tide has ebbed, and so on, day in and day out. 
The tidal zone is thus a very difficult habitat, and also a very compli- 
cated one because of differences in the timing and height of tide. 
Since low tide and high tide occur about 1 hour later each day, the 
intertidal area comes to be exposed at all hours of the day and night. 
During the first and last quarters of the moon neap tides occur, i.e., 
tides of low amplitude, but during periods of new moon and full moon 
the tides rise higher and fall lower and are known as spring tides. In 
some localities, such as the west coast of the United States, one set of 
high and low tides each day has a much greater amplitude than the 
other set. The height of the tide differs greatly from locality to lo- 
cality, and the duration and depth of inundation varies according to 
the level within the tidal zone at which a given organism exists. For 
example, an animal or a plant attached near the upper extreme limit of 
the tide will be covered by sea water for only a brief period about 
every two weeks, whereas an individual living near the lower extreme 
tidal limit will receive only a correspondingly short exposure to the air. 
The tidal zone comprises the upper portion of the littoral zone, as 
indicated in Fig. 2.4. 

Despite these severe conditions a dense population of a very wide 
variety of plants and animals characteristically inhabits the tidal 
zone. The fauna and flora consist chiefly of sessile forms, such as the 
common seaweeds, barnacles, mussels, and clams. Those plants and 
animals that cannot move out of the tidal area must endure the full 
effect of the differences in the two media. In addition, certain active 
forms forage here under favorable conditions and then retreat. At 
low tide animals come from the land and feed on the material that 
has been exposed. Shore birds are familiar in this area, but less 
observed are rats, mice, mink, skunks, and other small mammals that 
feed chiefly at night. During high tide the reverse situation obtains. 
Fishes, crabs, and other active animals from deeper water move in for 
a few hours and then retire. The alternating visitations of these forms 
remind one of Box and Cox in the famous operetta. 

It is of interest to inquire why the tidal zone should be so densely 



102 Water 

populated since the habitat presents such great difficulties for exist- 
ence. In the first place all species tend to spread; marine animals 
and plants are pressing on their boundaries, and terrestrial forms arc 
likewise attempting to invade regions at the margin of their habitats. 
These species extend their ranges just as far as they can endure the 
conditions. Furthermore, the tidal zone has certain advantages over 
conditions in deep water. Shallow water has more light, oxygen, and 
circulation; and certain predators are excluded. A great amount of or- 
ganic matter occurs in this zone, and much of this forms a valuable 
food for the inhabitants. A large part of the organic material exists 
as detritus resulting from the breakdown of seaweeds and the dis- 
integration products of dead animals. In addition many kinds of 
smaller planktonic organisms abound in the shallow water and add 
to the food supply. The adaptations of the intertidal organisms and 
the control of their activities by the environment will be discussed 
briefly here; more comprehensive treatment of this special habitat will 
be found in such books as Yonge (1949) and Wilson (1951). 

Practically all the permanent inhabitants of the tidal zone are 
aquatic organisms, and accordingly their chief problem is dealing with 
the adverse conditions of the air medium when the tide is out. Many 
physiological and morphological adaptations are displayed for with- 
standing the recurring periods of water shortage. Many of the 
simpler organisms can endure a considerable drying of their tissues 
without permanent injury. Sea anemones, for example, have been 
kept out of water 18 days; at the end of this period the animals looked 
like dried raisins but became active again when replaced in sea water. 
Attached forms with shells like the barnacles, mussels, and oysters 
simply close up shop during the period of low tide, and many can 
remain sealed up in this way for unbelievably long periods without 
injury. If kept at a low temperature oysters may be stored out of 
water for 3 months or more and will resume normal activity when 
replaced in the sea. Snails and starfish move into sheltered crevices 
when the tide is out; worms, clams, and other burrowing forms dig 
deeper in the mud and wait for the tide to return. 

Certain intertidal forms seem to benefit by periods out of water. A 
practical application of this fact is made use of by many oyster 
growers who prefer to raise oysters in situations where they are ex- 
posed to the air for at least a portion of each day. The oystermen 
believe that as a result the muscles which close the shells are strength- 
ened. When the oysters are shipped to market, the shells are held 
more tightly together thus retaining the fluids within the mantle 



Tidal Zone 



103 



cavity. These oysters are said to be better able to "hold their 
liquor!" 

Actively swimming or crawling animals may be able to go without 
food for the period when the tide is out, but they cannot remain 
active and do without a supply of oxygen for this length of time. 
There is no lack of oxygen in the air, of course the problem is to 
get it without allowing the respiratory membranes to dry out. The 
gills of many of these animals are placed in partially enclosed cavities 
of the body where oxygen may reach them but the loss of water from 
the respiratory surfaces is retarded. A striking gradation in the 
anatomical adaptations of crabs in the tidal zone has been described 
by Pearse (1950) for the North Carolina shore. He called attention 
to the fact that species living higher up on the shoreline show a re- 
duction in the number of their gills and particularly in the ratio of 
gill surface to the size of the whole animal (Fig. 4.3). 




Pearse, 1936 

FIG. 4.3. Reduction in number of gills and increase in ratio of body to gill 
volume in species of crabs living at progressively higher levels in the littoral zone. 

Differences in ability to withstand the progressively more difficult 
conditions from the low-tide mark to the high-tide mark have resulted 
in a zonation of the plant and animal species. On rocky shores this 
zonation is often so pronounced that it can be seen at a distance when 



104 



Water 



the tide is out. In the situation shown in Fig. 4.4 the sharp upper 
limit for the growth of the rock weeds is easily apparent. At a higher 
level the longer period of air exposure and of lack of sea water im- 
posed a clearly defined upper boundary upon the barnacle population. 




Photo by R. W. Miner 

FIG. 4.4. Rocky shore at low tide showing stratum of sea weeds (Laminarian 
Zone) and of barnacles (Balanoid Zone). 

Stephenson and Stephenson (1949) have found that on rocky shores 
in many parts of the world three fundamental zones may be recog- 
nized: the Littorina Zone, characterized by snails; the Balanoid Zone, 
characterized by barnacles; and the Laminarian Zone, characterized 
by laminarian seaweeds. The relation of these zones to the tide marks 
is shown in the diagram. 





Littorina Zone 




Balanoid Zone 


I 


^a mi nar Ian Zone 



High Spring Tide 



TIDAL ZONE 



Low Spring Tide 



Tidal Zone 



105 



The upper part of the Littorina Zone extends into the spray zone, or 
supralittoral zone, where the salt-resistant outposts of the land fauna 
and flora are to be found as well as those marine organisms most 
capable of enduring desiccation (Boyce, 1954). The Laminarian 
Zone extends for varying distances below low-tide mark. 



: v,-:^Wp:$^^ %%:*& 

'i$fcM?^^^ ; ^&^'& 

Aftr :' y/vv ,>':'? Jv *,**,*i*tY oiA**,*t* *%** <r ".. >\H- 




Stephenson and Stephenson, 1949, copyright Cambridge University Press 

FIG. 4.5. Diagram to illustrate the three fundamental /ones of lile between tide 
marks on a rocky shore. Balanoid '/one showing (A) barnacles only, (B) 
barnacles inhibited by sweep of seaweeds, and (C) al^al subdivisions typically 
encountered on British coasts. 

Frequently various subdivisions are superimposed on the main 
zonation of rocky shores (Fig. 4.5). The vertical limits of the various 
subzones of fucoid algae and of associated invertebrates are deter- 
mined not only by relative tolerance in respect to the water factor 
but also by type of rock, wave action, and relative success in invasion, 
competition, and resistance to predation. 



106 



Water 



On muddy and sandy shores zonation of plants and animals be- 
tween tide marks similarly occurs, but the observer will have to dig 
to ascertain the full ramifications of the tidal relationships since many 
of the inhabitants are burrowers. In addition, microscopic examina- 
tion of samples of bottom material reveal a zonation of the microor- 
ganisms in the tidal zone. The distribution of copepods dwelling in a 
sandy beach of Cape Cod will serve as an illustration ( Fig. 4.6 ) . The 



Paraleptastacus 
brevicaudatus 



Psammoleptastacus 
arenaridus 




-1.5 



1.5 3.0 4.5 6.0 7.5 

Horizontal distance above low-tide mark, m 



10.5 



Pennak, 1939 

FIG, 4.6. Abundance and distribution of sand-dwelling copepods across the tidal 
zone at Nobska Beach, Woods Hole. 

different species present were so nearly alike in general appearance 
that they could be distinguished only by careful study. Neverthe- 
less, one species was found to live only below low tide where it is never 
subjected to the drying action of air. Another form was most abun- 
dant midway between the tide marks. A third species was most 
numerous just below the high-tide line. Furthermore, when samples 
of sand from different levels beneath the surface were studied, the 
numerical abundance of the copepods was found to vary with depth 
in the sand as well as with position across the tidal zone. At the low- 
tide mark the largest number of animals was found in the upper 4 cm, 
whereas at the high-tide mark the largest population occurred at a 
depth of 12 to 16 cm. In the tidal zone, as in other amphibious situa- 



Water Problem in Terrestrial Environment 107 

tions, the water factor thus exerts a major controlling influence on 
the minute organisms as well as on the more conspicuous species. 

WATER PROBLEM IN THE TERRESTRIAL ENVIRONMENT 

When we turn to dry land as an environment, the water problem 
becomes extremely acute. Everyone is familiar with the quick death 
which awaits aquatic organisms brought out on land. Many terres- 
trial forms can endure the land environment only if they remain in 
damp places. This is true for a great many species of plants and 
also for animals without special protection. A salamander that has 
escaped from its terrarium and run across the dry floor will soon be 
dead if it is not rescued and put back in a humid atmosphere. 
Earthworms frequently crawl out on a concrete sidewalk during a 
spring rain. If the sun comes out and dries the sidewalk, many of 
the animals will be killed before they can find their way back to the 
moist ground. 

The successful invasion of the dry land is dependent upon the 
possession of methods for securing and retaining sufficient water and 
at the same time allowing adequate exchange with the environment. 
Terrestrial plants and animals as well as aquatic forms must maintain 
a proper water balance between water income and loss. Great varia- 
tion exists in the moisture content of land organisms and also in the 
amount of water intake and output that is necessary to provide for 
metabolic processes. Insects subsisting on dry food probably hold 
the record for the minimum water exchange. At the other end of the 
scale are many types of higher plants that are extravagant water 
users. Some species of plants absorb and transpire more than 2000 
grams of water for every gram of dry matter produced by assimila- 
tion. An oak tree may transpire as much as 570 liters of water in a 
single day. 

How many taxonomic groups have succeeded in colonizing dry 
land? Actually, only a very few of the prominent kinds of plants and 
animals have accomplished this. In the plant kingdom it is chiefly 
the vascular forms that are able to live in really dry places, but some 
low-growing bryophytes, algae, and fungiparticularly the rock lich- 
ensform noteworthy additions to the list. Among the vertebrates, 
only the reptiles, birds, and mammals can live freely exposed to dry 
air. Although several phyla of invertebrates are reported in the land 
fauna, only the insects, spiders, and snails occur in important numbers 
in dry habitats. By contrast, there are a great many plant and ani- 
mal groups that are exclusively aquatic. 



108 



Water 




Occurrence of Water in Land Environment 109 



Occurrence of Water in Land Environment 

The only ultimate source of water for the terrestrial environment 
is condensation chiefly in the form of rain. If precipitation were 
evenly distributed, it would cover the earth to a uniform depth of 
1 m each year. Quite the contrary is the case. A glance at Fig. 4.7 
will reveal the irregularity in the geographical distribution of rainfall. 
In the desert areas the rainfall is less than 25 cm per year and is in- 
adequate for most organisms. In the black areas of the chart an 
excessive rainfall of 200 cm or more is reported each year. The mean 
annual rainfall on the south side of the Himalayas is recorded as 1232 
cm. At the other extreme is Iquique, Chile, with an average precipi- 
tation of 0.125 cm per year. Among the Olympic Mountains in the 
state of Washington the annual rainfall totals as much as 380 cm on 
the windward side of some ridges and as little as 25 cm on the lee- 
ward side. 

Seasonal variations in the distribution of rain may be of even 
greater importance than the geographical aspect. In many regions 
the division of the year into a rainy season and a dry season is of more 
ecological significance than the change in temperature between sum- 
mer and winter. In the climatographs shown in Fig. 4.8 two situations 
are contrasted. In the Chicago region representing a temperate cli- 
mate, an average of 5 to 9 cm of rain falls in every month of the year, 
but the average monthly temperature varies from about 4C in 
the winter to about 22C in the summer. In the tropical climate of 
Barro Colorado Island the average monthly temperature changes by 
not more than 1 or 2C throughout the year, whereas the precipitation 
drops to less than 3 cm per month in the dry season and rises to more 
than 40 cm per month in the wet season. 

Wherever the amount of precipitation varies greatly during the 
course of the year, the time of occurrence of rain in relation to the 
temperature cycle has great effect on the vegetation. Only those 
species will thrive whose varying needs for water in the different life 
stages are satisfied by the seasonal distribution of available moisture. 
In many of the grass-covered or forested areas of the temperate zone 
the major portion of rain occurs in the summer, but in other regions, 
of which southern California is an example, most of the precipitation 
occurs in the winter. In the latter situation we find a special type 
of vegetation with broad evergreen leaves known as the sclerophyllous 
forest. In the prairie provinces of Canada the average annual precipi- 
tation of 50 cm would not be sufficient to support agriculture if the 



110 



Water 



rain were evenly distributed through the year. However, the major 
portion of the rainfall occurs during the spring growing season, and 
also at a time before the rate of evaporation has become excessive, 
with the result that the available precipitation is used to best advan- 
tage. The existing combination of ecological conditions thus permits 
western Canada to be an important wheat-growing region. 

The foregoing discussion brings into relief the fact that in consider- 
ing moisture conditions on land the actual amount of the precipita- 
tion is only part of the story. The other part, and often the more 



5.1 10.2 15.2 20.3 25.4 30.5 35.6 40.6 cm 

C 
-26.7 




1 23456 7 8 9 101112 1314 15 16 17 in. 

Average monthly rainfall 

FIG. 4.8. Average rainfall and temperature for each month of the year as indi- 
cated by the numerals. 

Lower figure: temperate climate data for Chicago, Illinois. 

Upper figure: tropical climatedata for Barro Colorado Island, 
Panama Canal Zone. (Reprinted with permis- 
sion from Hesse, Allee, and Schmidt, Ecological 
Animal Geography, 1951, John Wiley and Sons, 
New York.) 



Moisture in the Soil 111 

important part, is the loss of water. Of the rain that falls upon the 
surface of the soil, part runs off immediately, part sinks in, and another 
portion is lost by evaporation. The amount of moisture at any one 
time and place depends upon the relative rates of the supply of water 
to the soil and evaporation from itanother example of an ecological 
factor whose value depends upon an equilibrium. In evaluating the 
water factor in the terrestrial environment both supply and loss 
processes must be taken into account. 

Moisture in the Soil. Since the terrestrial environment is so varied, 
it is not surprising that the water factor is very complex, and the 
moisture in the soil and in the air will first be considered separately. 
When rain water enters the soil, it fills the spaces between the par- 
ticles. The volume that can be filled is known as the pore space and 
commonly varies between 60 per cent for heavy soils and 40 per cent 
for light soils. When the rain ceases, a certain portion of the water 
in the soil soon drains out, and this is known as the gravitational water. 
The portion of the soil water that is held by capillary forces around 
and between the particles is the capillary water. That part of the 
pore space which is not filled by gravitational or capillary water may 
be occupied by water vapor. Another form of soil water, termed the 
hijgroscopic water, occurs as an extremely thin film on the soil grains 
but this cannot move as a liquid. A small portion of the soil water is 
chemically bound with soil materials and is known as combined water. 
The total amount of capillary, hygroscopic, and combined water plus 
the water vapor constitutes the field capacity and is the maximum 
amount of water that the soil can hold after the gravitational water 
has drained away (Fig. 4.9). 

The maximum potential supply of water available for the vegetation 
is represented by the full field capacity of the soilf As plants draw 
on the soil water, they gradually reduce the amount held in capillary 
spaces. Below a certain moisture content plants tend to wilt in the 
hottest part of the day. When the capillary water has been still 
further depleted, a point is reached at which the plants will not re- 
cover from wilting until more water is added to the soil, regardless 
of other environmental conditions. At this point absorption of water 
by the plants has become too slow to replace the water lost by 
transpiration. The moisture then remaining in the soil is designated 
as the permanent wilting percentage, or the wilting^coefficient. _ Since 
little difference has been found in the abundance oi' soil water when 
wilting occurs for plants of various species growing in the same soil, 
the permanent wilting percentage is primarily a characteristic of the 
soil, and as such has great ecological significance. The amount of 



112 



Water 



water between the full field capacity, as a maximum, and the wilting 
coefficient, as a minimum, represents the available water. As indi- 
cated in Fig. 4.9, a considerable amount of water may still remain in 
the soil after the wilting coefficient has been reached. A fraction of 
the capillary water, the hygroscopic water, and the combined water, 
as well as the water vapor, cannot be obtained by the plant, and these 
constitute the non-available water. 

SOIL WATER 



Pore space- 



Field capacity 



Gravitational 



Capillary 



Available 



Wilting coefficient 



Hygroscopic 



Combined 



Non-available 



Fig. 4,9. Generalized diagram of the forms of soil water. The relative propor- 
tions and the value of the wilting coefficient differ according to the nature of 

the soil. 

f; 

Soils vary considerably in the relative proportions of the different 
categories of soil water. In some instances the amount of non-avail- 
able water may actually be larger than the available water. The 
fact that a portion of the water in the soil is unavailable to land or- 
ganisms and that water in the sea is unavailable to aquatic organisms 
not adapted to its osmotic pressure contributes further to the general 
concept of the universality of the water problem. From the ecologi- 
cal point of view the actual amount of water present in any habitat is 
not as^ important as its availability. 

Moisture in the Air. For many terrestrial organisms the moisture 
in the soil is chiefly important as constituting the principal source of 
water, whereas the moisture in the air is mainly significant as con- 
trolling the loss of water. For this reason the absolute humidity, or 



Moisture in the Air 



113 



total amount of water hi the air. is generally of far less ecological 
consequence, than the relative humidity. The ecologist therefore 
focuses his attention on the relative humidity or the amount of mois- 
ture in the air as a percentage of the amount which the air could hold 
at saturation at the existing temperature. Since the capacity of air 
for water vapor increases with temperature, the relative humidity of 
the atmosphere is reduced in any situation in which an increase of 
temperature occurs without an accompanying increase in the total 
moisture content of the air. 

In the terrestrial environment the ecological effect of the water 
factor is consequently strongly influenced by the temperature factor. 
Of two regions having the same rainfall the warmer is the drier in 
the ecological sense (Fig. 4.10). The climate of a locality with a 



10 16 21 27 




in. 



cm 



0F 10 20 30 40 50 60 70 80 
Mean annual temperature 

Modified from McDougatt, 1925 

FIG. 4.10. Schematic representation of the influence of rainfall and temperature 

on climate. 



114 Water 

mean annual precipitation of 50 cm would be characterized as humid 
if the mean annual temperature were 7C or less. On the other 
hand, another locality with the same rainfall would be regarded as 
having a semiarid climate if the annual temperature were 21C or 
more. The contrast in climate between the Canadian prairie and 
the Mexican desert, both with about 50 cm of rain but with very 
different temperature conditions, is an excellent illustration of this 
principle. 

The geographical variation in relative humidity is very great. Rela- 
tive humidities of 80 to 100 per cent characterize the tropical rain 
forest. Regions reporting values of less than 50 per cent are regarded 
as having dry climates, and those with values of less than 20 per cent 
arc extremely arid. It is of interest to note in passing that in cold 
winter weather the relative humidity inside our houses is rarely higher 
than 35 per cent. 

At any one locality the relative humidity may remain relatively con- 
stant for long periods of time or may vary widely. On many oceanic 
islands the humidity is very nearly the same throughout the year. 
In other localities, characterized by wet and dry seasons, the humidity 
fluctuates widely from one part of the year to another. In certain 
situations as on the plains and in desert regions considerable changes 
in moisture content may occur during the course of each day. Records 
made in a short-grass prairie in the United States showed a variation 
from a relative humidity of less than 30 per cent in the early after- 
noon to more than 95 per cent in the middle of the night (Fig. 4.11). 
Obviously animals and plants living in such habitats must be equipped 
to withstand these rapid and extensive changes in the water factor, 
and their lives must be attuned to them. In deserts where daytime 
humidities are extremely low and evaporation is excessive, the greater 
relative humidity at night helps to relieve the critical moisture con- 
dition. Many desert animals take advantage of this situation by 
going abroad only during hours of darkness; in some desert plants the 
stomata open only at night when transpiration loss is at a minimum. 

The foregoing has been a brief description of the amount of moisture 
in the air in terms of its relative humidity. As already implied, the 
chief ecological significance of the relative humidity is its effect on 
the rate of water loss. Terrestrial animals and plants lose water 
directly to the air by evaporation and transpiration. The water 
supply in their substratum is also reduced by direct evaporation from 
the soil and indirectly by the transpiration of the vegetation. Evap- 
oration takes place very rapidly from soil because of the great surface 
area presented by the fine particles and causes the drying of the upper 



Moisture in the Air 



115 



layers. Water is lost from the deeper layers chiefly by absorption by 
roots and attendant transpiration. 

The magnitude of water loss by transpiration is often not appre- 
ciated. Perhaps some idea can be obtained from the fact that a 
hectare (2% acres) of mature oak trees will transpire as much as 
25,000 liters of water per day. This volume would be equivalent to 
a layer of water 0.25 cm deep over the whole area. In other words, 
if tra'nspiration continued at this rate for 10 days, a rainfall of at least 
2.5 cm would be required to restore the water. One might think that 
the maximum rate of water loss would occur from saturated soil or 
from a pond. Actually the rate of water loss per unit area from the 
ground due to the transpiration of the vegetation may be nearly twice 
as great as that from a free water surface due to evaporation. 



100 



40C 




18 



12 18 6 12 18 6 12 
Thursday Friday Saturday 

FIG. 4.11. Hygrothermograph record of temperature (broken line) and relative 

humidity (solid line) in the short-grass plains of central United States during 3 

days in early July. ( By permission from Plant Ecology by Weaver and Clements, 

1938, McGraw-Hill Book Co.) 

Three atmospheric conditions that greatly modify the rates of evap- 
oration and transpiration are the saturation deficit, the temperature, 
and the wind velocity. The saturation deficit is the difference be- 
tween the actual vapor pressure and the maximum possible vapor 
pressure at the existing temperature. The saturation deficit thus gives 
more information of ecological significance than the relative humidity 



116 Water 

alone since the saturation deficit provides a measure of the capacity 
of the air to take up additional moisture. An increase in the satura- 
tion deficit produces a rise in evaporation rate. 

An increase in temperature similarly speeds up the evaporation 
process. A good indication of the magnitude of this influence is ob- 
tained from the amounts by which reservoirs are lowered in regions 
of different temperatures. During one year the water level in a 
reservoir in Ontario dropped 38 cm, a reservoir in California lost 2.4 
m, and one in Egypt was lowered 3.6 m through evaporation alone. 
The distribution of plants often reveals the influence of temperature 
on direct and indirect water loss. On mountain slopes vegetation 
zones are found at different altitudes according to the exposure to the 
heat of the sun. Moisture-demanding species are restricted to 
higher levels on the side of the mountain toward the equator than they 
are on the cooler opposite side. The variation in the amount of rain 
required for a good growth of short grass furnishes another illustra- 
tion of the effect of temperature. In Montana a precipitation of 
about 35 cm is sufficient, but in northwestern Texas a rainfall of 53 
cm is needed to produce the same amount of grass. The explanation 
of this fact is easily apparent when it is realized that evaporation in 
the 6 summer months ifl Montana amounts to 84 cm, whereas in 
Texas it averages 137 cm (Weaver and Clements, 1938). 

The wind velocity also exerts a major effect on the loss of water. 
With a gentle zephyr of only 8 km per hr the transpiration of plants 
is increased 20 per cent over that which they exhibited in still air. A 
wind velocity of 16 km per hr increases the transpiration by 35 per 
cent and one of 24 km per hr increases it by 50 per cent. The desic- 
cating action of warm dry winds sometimes prevents the invasion of 
windward mountain slopes by vegetation that grows perfectly well on 
the leeward sides of the same slopes where water loss is much re- 
duced. The harmful effect of excessive evaporation caused by the 
wind may also be seen in one's own garden. Here winter killing is 
occasionally due not to very low temperatures, but to a combination 
of wind and high temperatures! If a warm, dry wind blows for too 
long a period when the ground is frozen, the plants may lose water 
faster than their roots can obtain it and they will succumb as a result. 

Our discussion of moisture in the soil and in the air has indicated 
that living organisms particularly the higher plants may exert a pro- 
found reciprocal influence on the water conditions in the terrestrial 
environment. Water is a highly modifiable factor in land habitats, 
and living elements may act either to augment or to diminish the 
amount of moisture present. Vegetation tends to catch the rain, so 



Microclimates 117 

that, if the precipitation comes in short showers, the rain may never 
reach the soil. The plant cover also tends to reduce the water con- 
tent of the deeper layers of the soil as a consequence of its transpira- 
tion, but at the same time it adds moisture to the air. On the other 
hand, the presence of forest vegetation favors the reduction of evap- 
oration at levels near the ground because it lowers the temperature, 
slows down the wind, and sometimes causes increased condensation. 

The amount of water present at any point in a land habitat and the 
rates of gain and loss are thus seen to be the result of the equilibrium 
of processes both climatic and biological. The interrelations of the 
water factor and the vegetation are frequently very complicated and 
are the subject of elaborate studies beyond the scope of this book. 
For further information the reader is referred to such treatments as 
Daubenmire (1947, Ch. 3). In addition to presenting the general 
picture, the present discussion reveals the interdependency of vegeta- 
tion and moisture as another outstanding example of the organism and 
the environment acting as a reciprocating system. 

Microclimates 

In the foregoing discussion of the variation in water loss under 
various conditions of humidity, temperature, and wind the values 
given were often those characteristic of extensive areas. Very fre- 
quently, however, the values of these factors in the immediate sur- 
roundings of the organism differ markedly from the regional values. 
In other words, the immediate climate of the organism is often sharply 
different from the average climate of the region as reported by stand- 
ard meteorological records. The realization of this fact has led to 
the development by ecologists of the concept of the microclimate. In 
considering the effect of climatic influences on plants and animals it is 
absolutely essential to measure the environmental factors as they ac- 
tually reach the organism. The effective climate, upon which the 
life of the organism depends, is its microclimate. The characteristics 
of the microclimate may, or may not, be similar to the regional climate 
(Geiger, 1950). 

Local variations in climatic factors of concern to many organisms 
may involve distances of several meters in the surroundings, but the 
microclimates of forms living in the soil, in rock crevices, under the 
bark of trees, or in similar confined habitats often have dimensions 
measured in fractions of a centimeter. The concept of microclimates 
is particularly important in relation to moisture because relative 
humidity varies widely within short distances in any irregular habitat. 



118 Water 

In swampy woodland, for example, measurements have shown that 
a high rate of evaporation characteristically exists near the tops of 
the trees. At half this height the evaporation rate was reduced to 
30 per cent in one instance. Lower down, near the damp soil, the 
animals and plants were subject to an evaporation rate of only 7 per 
cent of that near the treetops. We are consequently not surprised to 
find that this vertical change in moisture, as well as vertical gradients 
in light and in other factors, contributes toward the establishment 
of a characteristic vertical layering by species of the vegetation and 
sometimes also of the arboreal animal life in forests. Among animals 
the difference in evaporation rate with distance above the v ground is 
of particular importance to soft-bodied insects. Many of these are 
known to move higher in the vegetation at night, but during the day 
they are forced by excessive evaporation to retreat to lower levels. 

Contrasting values for the rate of water loss in two microclimates 
were reported in an interesting experiment performed in the Middle 
West. Two sets of green ash seedlings were planted in pots; one set 
was placed in the midst of a sumac thicket, and the other set was put 
out on the open prairie only a short distance away. At the end of 8 
days the first group of seedlings was found to have lost an average of 
0.38 grams of water per sq cm of leaf surface. The seedlings set out 
in the adjoining prairie had lost 0.88 grams of water per sq cm, or 
more than twice as much during the same period. 

A traditional method of finding the approximate direction of north 
in the woods is by observing on which side of tree trunks the growth 
of green "moss" is thickest. In the North Temperate Zone in habitats 
in which the atmospheric moisture is near the critical point, algae and 
other non-vascular plants are characteristically more abundant on the 
north surfaces of tree trunks, because the higher evaporation due to 
insolation effectively retards growth on the south side. However, in 
many situations differences in exposure to wind may outweigh the 
effect of the sun in causing excessive water loss. Thus, although the 
"north side" rule does not always hold, direct measurements show 
that the growth of epiphytic moss is a very sensitive indicator of micro- 
climate, and the striking differences in the growth of algae on op- 
posite sides of tree trunks can be seen by anyone who walks through 
the woods. An average value for the moisture condition of the forest 
would give no inkling of these small-scale ecological differences of 
crucial importance to the species concerned. 

The burrows of desert animals furnish another example of a micro- 
climate in which the physical conditions are radically different from 
those of the region as a whole. When the kangaroo rat retreats 



Meeting Water Problem on Land 119 

within its burrow, it breathes air two to five times more humid than 
that outside. Measurements indicate that, if this animal remained 
out of its burrow continuously, the rate of evaporation from its lungs 
would exceed the rate at which it could obtain water, and the re- 
sulting water loss would eventually cause death. Temperatures in 
desert burrows have been found to be as low as 28C at the same time 
that values for the surface of the soil surpassed 71C (cf Table 9). 
Such critical differences in temperature and in other factors in micro- 
climates will be considered further in subsequent chapters. 

Meeting Water Problem on Land 

Land plants can live only where sufficient water reaches them either 
through the air or through the soil. Some species can get along with 
relatively small amounts of water, and these plants, inhabiting deserts 
and other arid regions, are termed xerophytes. Plants eke out an 
existence on a reduced water supply in a variety of ways. Certain 
annual plants are adapted to maintain themselves in desert areas by 
remaining in the seed stage during the dry season and then passing 
rapidly through their life cycles when a rainy period occurs. Other 
desert plants, like the cacti, store water in succulent organs in suffi- 
cient quantities to tide them over long intervals of drought. In addi- 
tion, various species of non-succulent perennials, such as the creosote 
bush, sage brush, and many grasses are included among the xerophytes 
because they possess unusual ability to endure long periods of per- 
manent wiltingin some instances running into years. Lichens and 
some mosses similarly can survive a surprising degree of drying out. 
Plants with water relations intermediate between those of the xero- 
phytes and the hydrophytes are classified as mesophytes. 

Since land animals can move about, they can actively seek pools 
of water from which to drink or can obtain water in the form of snow 
or dew. Many are able to endure long periods between visits to the 
water supply. Some terrestrial animals, such as frogs and toads, 
have the ability to absorb water through their skins from damp sur- 
roundings, whereas others find sufficient liquid water in the tissue 
fluids of their food. Another method of solving the problem of 
securing water in dry land habitats, and one employed widely by the 
insects, is the use of metabolic water, that is, the water resulting from 
the chemical breakdown of food materials. The clothes moth and 
the meal worm have the ability to obtain metabolic water in sii%sient 
quantity to enable them to live their whole lives without any addes$ 
to free water in the environment. Experiments indicate that the 



120 Water 

kangaroo rat of the Arizona desert may similarly secure the whole of 
its water supply as a metabolic by-product of its food (Schmidt- 
Nielsen, 1950). 

Just as important as methods for obtaining water are means for 
resisting desiccation. Many of the same general types of adaptations 
having this effect have appeared in the course of evolution amongst 
both plants and animals. First and most obvious is the possession 
of a more or less impervious covering for the body. The cuticle of 
plants, the skin of birds and mammals, the exoskeleton of insects, and 
the mucous secretion of mollusks are examples. Another means for 
avoiding the excessive loss of water is the reduction of body surface. 
The leafless plants of the desert are an instance of this plan carried 
to an extreme. Other plants fold their leaves or turn them edgewise 
to the scorching heat of the sun. Many terrestrial invertebrates 
bury part or all of their bodies to reduce evaporation. It is well 
known that the termites which attack our houses are killed by exposure 
to dry air, but these pests can cross exposed foundations by construct- 
ing runways within which protection from excessive evaporation is af- 
forded. In arid regions many of the smaller animals are active only 
during the night when drying is less intense. 

Water may also be saved by reducing the loss in respiratory and 
excretory systems. Respiratory organs are often enclosed within a 
cavity as has already been mentioned in relation to the gills of land 
crabs. Lungs are commonly located well within the body with only 
a small opening to the exterior. The branching tubular tracheal sys- 
tem of the insects is another example, and in many forms the size of 
the external openings of the tracheae may be reduced by the spiracles. 
In a similar way evaporation from the internal cavities of the plant 
leaf is restricted by the guard cells of the stomata. 

Reduction in the amount of liquid needed for excretion is a physio- 
logical adaptation that also aids the body in conserving water. Many 
animals with little available moisture in their habitats and in their 
food produce dry faeces. Furthermore, in the interest of saving 
water, it is desirable for land dwellers to dispose of their nitrogenous 
wastes with the minimum quantity of liquid urine. Mammals ac- 
complish something in this direction by reabsorbing part of the water 
in the urine before it is excreted. The ultimate attainment in this 
matter is reached by those birds, reptiles, and insects that are able to 
excrete nitrogenous wastes as solid uric acid practically without 
water loss. 

If lack of water becomes still more acute, it may be necessary for 
the organism to go into a dormant condition in which its need for 



Influence of Moisture on Growth and Distribution 121 

water and its loss of water will be reduced. In the tropics many 
trees shed their leaves during the dry season. Other plants pass 
periods of drought as seeds or as spores. Many lower animals form 
cysts of one sort or another. Terrestrial mollusks close their shells 
with a thin epiphragm of secreted mucus, and more highly organized 
animals, such as the ground squirrels, go into a state of torpor in 
their burrows. Dormancy of this sort occurring under conditions of 
heat and drought is referred to as aestivation, and is found character- 
istically in desert and tropical communities. Using various combina- 
tions of the methods for procuring and conserving water discussed 
above, plants and animals in arid habitats may live for months or 
years without rain or access to free water. 

Influence of Moisture on Growth and Distribution 

The growth of plants is often more directly and obviously affected 
by the water factor on land than is that of animals. The availability of 
moisture influences not only the rate of plant growth but also the 
growth form. Since the roots of most land species will not grow into 
saturated soil nor into soil devoid of available water, plants will be 
shallow rooted either if the water table is high or, in the contrasting 
situation, if the soil water is limited to the uppermost layer. In the 
latter case growth will be confined to the rainy season. The growth 
of plants whose roots extend to the permanently moist subsoil are 
largely independent of rain periodicity. 

Competition for water and the horizontal spread of roots often con- 
trol the spacing of plants in regions where water shortage is critical. 
The extent of the root system in a typical cactus is shown in Fig. 4.12. 
The roots are seen to be spread widely just beneath the surface of the 
soil where they can absorb whatever rain falls before it evaporates. 
This cactus will capture all the available moisture within the area 
shown and prevent any other perennial plant from gaining foothold 
in close proximity. Desert vegetation is often spaced out in a strik- 
ingly regular pattern as a result of this intense root warfare for water. 

The water factor also controls the geographical distribution of 
plants on both small and large scales. Such trees as willows and cot- 
ton woods are characteristically limited to the moist banks of water 
courses because their seeds will not survive more than a few days 
unless the soil upon which they fall is wet. The seeds of other species 
can remain dormant in a completely dry condition for months or years 
but will germinate only when sufficient water is available in the soil 
immediately surrounding them. In other instances the critical period 



122 



Water 



for the survival of a plant in relation to its water need may occur 
during adult life. Plants differ greatly in the amounts of water that 
they must absorb from the soil and transpire. Those with a high 
water requirement, or transpiration ratio, are limited to habitats where 
the supply of moisture is adequate. 




FIG. 4.12. Top view of surface roots of cactus plant shown on a 30-cm grid. 
Almost the entire absorbing system of roots occurred in the upper 2 to 10 cm of 
soil. (By permission from Plant Ecology by Weaver and Clements, 1938, Mc- 
Graw-Hill Book Co.). 



Influence of Moisture on Growth and Distribution 123 

Differential ability to withstand drought will also play its role in 
controlling geographical distribution. If the soil moisture drops be- 
low the permanent wilting coefficient, most annuals and other tender 
plants will die within a matter of hours or days unless the soil water 
is renewed. On the other hand, succulents and plants that can 
aestivate are able to withstand varying periods of drought extending 
into years with some species. Thus the duration of drought periods 
is of crucial importance in determining whether a region may be in- 
habited by different plant types. 

Local differences in the moisture factor were clearly shown to in- 
fluence the species composition of the forest vegetation in a study 
made in Indiana (Potzger, 1939). A series of observations in a line 
running from north to south over a ridge revealed the fact that soil 
moisture was 20 per cent higher on the north side of the ridge than 
on the south side. Moreover, the rate of evaporation was more than 
twice as great at stations on the south slope than at those on the north 
slope. Quadrat counts of the number of trees on the two sides of the 
ridge gave the tabulated results. Although other ecological factors 

Species North Slope South Slope 
Maple 202 31 

Beech 112 

White oak 80 

Black oak 70 

Hickory 4 44 

undoubtedly played some part, moisture conditions were chiefly re- 
sponsible for the very pronounced differences in the occurrence of 
the various species on the two sides of the ridge. 

Another example of the control of distribution by the moisture fac- 
tor, and one on a much larger scale, is found in the regional occurrence 
of the natural vegetation in the United States. A glance at Fig. 4.13 
will reveal the fact that in the central part of the country, leaving 
out of account the irregularities due to the mountains, the zones of 
principal vegetation types tend to run north and south. In the eastern 
part of the continent the typical natural vegetation is forest ( in areas 
where man has not interfered). West of the forest zone is a belt of 
tall-grass prairie, and beyond that, running from Montana and North 
Dakota down to Texas is a zone of short-grass rangeland. Still farther 
westward lies the desert belt. These fundamental zones are indicated 
schematically in Figure 4.14. 

This zonation is obviously not controlled primarily by temperature 
since the temperature belts are seen to run generally east and west, 



124 



Water 





'"_''."" 
' 



Modified from the Graphic Summary of American Agriculture, U. S. Department of Agriculture 

FIG. 4.13. The distribution of the natural vegetation in the United States. 

if we again leave out of account the mountain and coastal regions 
(Fig, 5.14). Similarly, no good correlation will be found between 
the occurrence of these principal zones of vegetation and the annual 
precipitation taken by itself. As already explained, regional dif- 
ferences in evaporation must also be considered, and water loss- in 
turn depends in part on temperature. Both the supply and the loss 




.. ,. 

FIG. 4.14. Schematic representation of the four fundamental vegetational zones 
in central United States (from This is Our World by Sears, 1937, Univ. 1 of 

Oklahoma Press). 



Influence of Moisture on Growth and Distribution 125 

aspects of moisture conditions may be taken into account by calculat- 
ing precipitation-evaporation ratios for the "frostless season," i.e., the 
growing season for plants between the last spring frost and the first 
autumn frost. The pattern of the distribution of these ratios in the 
United States (Fig. 4.15) shows a general agreement with the oc- 
currence of the major vegetation zones. The natural forest vegeta- 
tion of the eastern part of the country occurs where the precipitation- 
evaporation ratios are high. In the Southeast and Northeast, the P-E 
ratio exceeds 100 per cent; that is, more rain falls during the course 
of the average frostless season than evaporates. Incidentally, this 
fact accounts in part for the excessive amount of soil leaching in these 
regions. In the Middle West precipitation-evaporation ratios be- 
tween 80 and 100 per cent exist. Values below 100 per cent indicate 
that the potential rate of water loss is greater than the rate of water 
supply. The line representing a P-E ratio of 60 per cent runs almost 
due south from the Dakotas to the southernmost point of Texas. It 
will be noted that this line agrees closely with the division between 
the tall-grass prairies and the short-grass rangelands. The 60 per cent 
line also approximates the position of the division between the pedo- 
cals and the peclalfers discussed earlier. P-E ratios .of less than 20 
per cent are found in the southwestern states, and their occurrence 
corresponds to the distribution of the desert vegetation. 

In view of the modifying influence on plant distribution of such 




tt ftt 

-, 



Modified from Livingston and Shreve, 1921 

FIG. 4.15. The precipitation-evaporation ratio indicated as a percentage for the 
average frostless season in the United States. 



126 Water 

factors as soil type and temperature and of the interdependence of 
these factors with moisture it is clear that no simple relation between 
the type of vegetation and any single physical influence can be ex- 
pected. However, the broad correspondence between the zones of 
vegetation and the availability of moisture will suffice to emphasize 
the major role of the water factor in the region under discussion in 
controlling both the soil and the plant life that develops with it. 
The application of the P-E ratios to these distributional problems has 
again brought into relief the fact that the balance between supply 
and loss is the most crucial aspect of the moisture factor for the vege- 
tation. 

The water factor on land also seriously affects the growth and dis- 
tribution of animals either directly or indirectly. Since the range of 
land animals is profoundly influenced by the vegetation, moisture 
often exerts its greatest effect on animals indirectly through its con- 
trol of the plants. For the higher vertebrates temperature is generally 
a more important direct environmental influence than moisture. 
However, it has long been known that races of birds and mammals 
in warm humid regions tend to be darker in color than races inhabit- 
ing the cooler and drier parts of the geographical range of the species. 
This generality is known as Glogers rule, but many exceptions exist 
and the relative effects of heat and moisture are not known. There 
is evidence that humidity acts more through the color of the soil as a 
background for the animals than in a direct way. The implications of 
Gloger's rule will be discussed in more detail below in relation to 
temperature and light. 

For amphibians and for insects and other terrestrial invertebrates 
moisture is often of great direct importance. Many insects exhibit 
critical dependence upon humidity conditions with sharp limits of 
tolerance. For example, the "silverfish" (Lepisma saccharina), a 
common household insect pest, finds optimum conditions for repro- 
duction at relative humidities of 85 to 90 per cent. The newly 
hatched nymphs die if they are subjected to relative humidities of less 
than 70 per cent, and they also succumb if their surfaces once become 
wet. The relationship of inserts to moisture is frequently very com- 
plex and varies greatly in different species and sometimes even in 
different parts of the life cycle of the same species (cf. Chapman, 
1931, Ch. 4). In the development of the flour beetle Tribolium the 
larval stage is accelerated by an increase in relative humidity, but the 
duration of the egg and pupa stages is unaffected by wide changes 
in this factor (Fig. 4.16). Moisture produces entirely different ef- 
fects upon the survival of this insect. In the larval stage increasing 



Influence of Moisture on Growth and Distribution 127 

humidities tend to improve survival, but the egg and pupa stages are 
affected to only a slight extent by relative humidities up to 70 per 
cent. However, at humidities above 80 per cent the viability of eggs 
and pupae drops very sharply. 

When the moisture conditions of the environment become adverse, 
the animals concerned either die or must migrate longer or shorter 
distances to more favorable locations. Droughts greatly reduce the 
occurrence of some species temporarily, or for long periods, by deci- 
mating the population. Locomotor activity, as well as feeding and 
other reactions, are controlled by humidity in certain animals, espe- 
cially insects. It is known that in some instances increased activity 
without orientation results from a change in moisture conditions; in 



HU 
on 




^ 














to Larvae 


^^^^ 




i n 






Pupae N 








j 




__v^ 






n 


' 


Eggs-^" 




- -- 





1UU 

90 
80 
70 
60 
50 
40 
30 
20 
10 

c 


1 


i -- 

^r^"" 


^Pupae-^"* 


'^J-L^ 

:^ 


1 

ae 
^Eggs- 






- 








\ 


- 








V 


- 








|\ 
\ 


1 


i 


1 


I 


i 
i 

1 I 


) 10 20 30 40 50 60 70 80 90 1C 
Relative humidity 



Fig. 4.16. Duration (upper) and survival (lower) of the indicated stages of 
Tribolium confusum at different relative humidities under constant temperature 
of 27C (by permission from Animal Ecology by Chapman, 1931, McGraw-Hill 

Book Co.). 



128 Water 

other instances animals are able to respond clirectionally to a humidity 
gradient. However, the whole problem of control of locomotion by 
moisture in land animals calls for further investigation. 

The discussion of water relations in this chapter emphasizes the 
universality of the water problem. Plants and animals in aquatic, 
amphibious, and terrestrial situations are faced with the difficulty of 
providing for water exchange and at the same time maintaining the 
proper water balance. The amount of water present in the environ- 
ment and its transfer into and out of the organism are results of an 
equilibrium of forces. In the terrestrial environment the organisms 
themselves, especially the plants, can affect the amount of moisture in 
their surroundings. The total amount of water is not of as great 
significance as its availability. In some situations the environment 
tends to extract water from the plant or animal at an alarming rate. 
If the organisms cannot obtain water fast enough and retain it in suf- 
ficient quantity, the lack of this essential material becomes a critical 
limiting factor. 



5 

Temperature 



Temperature is perhaps the most commonly familiar ecological fac- 
tor. The great variation in heat conditions and their general influence 
are evident to everyone. In contrast to many ecological factors tem- 
perature may be measured with relative ease. Temperature is a uni- 
versal influence and is frequently a limiting factor for the growth or 
distribution of animals and plants. Even when an organism is dor- 
mant, the chemical processes going on in its body are controlled by 
the existing temperature. This factor is also important indirectly as 
modifying the effects of other ecological agents. 

Temperature is the intensity aspect of heat energy. The capacity 
aspect of this form of energy will also be considered in this chapter. 
Differences in the heat capacity of the media, of the various types of 
substrata, and of the bodies of organisms themselves are significant 
in controlling susceptibility to temperature change. The amount of 
heat in an ice-covered lake may change considerably without any 
alteration of temperature, but in most natural situations increase or 
decrease in heat also affects the temperature. Both aspects of heat 
energy must be kept in mind, but temperature has the more predom- 
inant direct influence on the lives of organisms. 

DISTRIBUTION OF TEMPERATURE 

Extremes of Temperature and of Tolerance 

In considering heat conditions in -natural environments the extremes 
of temperature may first be reviewed. In the open water of the 
aquatic environment the temperature cannot drop below the freezing 
point. This means that the ternperature of the water in ponds is 
never lower than 0C and in the ocean never lower than about 

129 



130 Temperature 

2.5C. The maximum temperature in marine environments of any 
size is probably represented by records of 36C in the Persian Gulf. 
In tide pools of the littoral zone and in shallow bodies of fresh water 
temperatures may go higher, 

On land the record for the lowest temperature is held by a locality 
in the north interior of Siberia where the thermometer was read at 
70C ( 93.6F) in 1947. Temperatures almost as low were re- 
ported for a northern outpost in the Yukon territory. At the other 
end of the scale air temperatures ranging above 60C (140F) are 
recorded in desert areas. Desert soils have been found to rise as high 
as 84 9 C when exposed to the noonday sun. The water in hot springs 
and geysers may approach 100C, and even higher temperatures occur 
sporadically in the very special situations presented by volcanic areas. 

Are these environmental temperatures beyond the ranges that can 
be tolerated by animals and plants? Birds and mammals are warm- 
blooded animals, or homoiotherms, that maintain their own constant 
internal temperature, and their tissues are insulated from the heat or 
cold of the outside world. All other animals are cold-blooded forms 
or poikilotherms. The tissues of these animals and of all plants, which 
are also poikilothermous, tend to approach the temperature of their 
immediate surroundings and to vary with external thermal conditions. 
No organism can continue to live in an active condition at tempera- 
tures below those at which its tissues freeze. As we shall see below, 
the freezing points of living tissues differ widely, but no poikilother- 
mous animal or plant can actually grow at continuing temperatures 
lower than a few degrees below 0C. At the other end of the thermal 
scale we find certain blue-green algae and thermophilous bacteria 
growing happily in hot springs, such as those in Yellowstone Park, at 
80 to 88C! However, relatively few poikilotherms can live perma- 
nently at temperatures above about 45C, and Brues (1927, 1939) 
found no reliable records for poikilothermous animals above 52C. 

It appears from the foregoing that the whole range of temperature 
in the sea is within the limits of tolerance of many plants and animals. 
In contrast, land temperatures may be far below or far above the 
temperatures that can be withstood by organisms in an active condi- 
tion. Since the range of heat conditions sustained successfully by in- 
dividual species is very much less, temperature extremes present a 
problem on both land and sea. The homoiothermous animals form a 
special case since the thermal range that they can tolerate is generally 
much greater than that of other animals and of plants, but even this 
group has definite limits of toleration. We shall be concerned, then, 
with the limiting action of extreme temperatures, as well as with the 



Changes in Time 131 

controlling action of intermediate temperatures for all kinds of plants 
and animals. 



Changes in Temperature 

Changes in Time. Let us consider first the temperature changes to 
which an animal or plant may be exposed if it remains in the same 
place. Time changes in temperature are controlled by various astro- 
nomical and climatic cycles. Within a few hours the sun may move 
so as to change the exposure of a sessile form from direct sunlight to 
shade. On land a very great difference in the heat received may oc- 
cur within a short period of time. In the water environment the 
change from sun to shade produces only a minor effect less than 
0.1C at a depth of 5 m. 

Similarly, the diurnal fluctuation in temperature is very much 
damped in the aquatic environment. In a body of water of any con- 
siderable size differences between day and night are commonly less 
than 1C. The maximum diurnal change in the ocean is about 4C, 
and with increasing depth the amplitude is reduced. Probably no 
diurnal temperature change is detectible below a depth of 15 m. 

Time of day 

8 12 16 20 24 4 8 12 16 20 24 4 8 12 16 20 24 




Modified from Weaver and Clements, 1938 

FIG. 5.1. Portion of a soil thermogram made in June on the prairie near Lincoln, 
Nebraska, showing the temperature fluctuations at depths of (A) 7.6 cm and 

(B) 30 cm. 

The temperature of the air near the surface of the land is sometimes 
17C higher in the daytime than at night, and in desert localities this 
difference may be as much as 40C. A still greater diurnal range in 
temperature is reported for the surface of the soil in desert areas. In 
every situation the amplitude of the change from day to night in the 
soil is reduced with depth. The soil thermogram shown in Fig. 5.1 
illustrates this point and also reveals the lag in the time of occurrence 
of the maximum and minimum daily temperatures. At a depth of 



132 Temperature 

only 30 cm the highest temperature of the day occurred near midnight 
and the lowest temperature occurred near noon! 

In the tidal zone changes in temperature are the result of a com- 
bination of the differences in the amount of heat delivered from the 
sun and of differences in the temperature of the air and of the water. 
AH organisms living between tide marks are subject to rapidly chang- 
ing temperatures that may test their tolerance to the limit. At one 
locality on the coast of Maine the temperature of the mud exposed at 
low tide was observed to rise to 38C under the midday summer sun. 
A short time later plants and animals living in this area were flooded 
by the incoming tide with a temperature of 10C. In the winter 
when the tide is out these organisms might be exposed to tempera- 
tures as low as 25C. The occurrence of these extreme and rapid 
fluctuations in temperature adds to the rigorous conditions of the tidal 
zone. 

Plants and animals are also concerned with seasonal cycles of tem- 
perature. In both tropical and polar seas the intensity of heat does 
not vary by more than about 5C throughout the year. In the tem- 
perate seas, however, changes of 10 to 15C from summer to winter 
are common, and sometimes seasonal differences of 23C or more are 
observed. At increasing depths in the water these differences become 
less. Measurements off Portsmouth, New Hampshire, where there 
was a 15-degree change in the surface temperature showed no meas- 
urable variation from summer to winter below a depth of 140 m. 
Probably more than 95 per cent of the oceanic environment exhibits 
no seasonal change in temperature that is of significance to living or- 
ganisms. In lakes and ponds seasonal fluctuations are generally 
greater than those found in the sea. Springs, however, form a very 
special fresh-water habitat in which the temperature may vary by only 
a degree or two the year around. A typical example of the thermal 
cycle in a lake showing the reduced amplitude in the deeper layers is 
presented in Fig. 5.2. 

On land the seasonal changes in temperature are familiar to every- 
one and are almost always of ecological significance. The greatest 
variations are found in continental areas in the temperate zones with 
lesser seasonal ranges in the tropics and near the coasts. At St. Paul, 
Minnesota, the average difference between summer and winter is 
33.7C. Possible extremes in range are even greater. From Tibet 
comes a report of temperatures varying by 77C from 37C in the 
winter to -f 40C in the summer. In contrast to the foregoing are 
certain localities where temperatures vary only slightly throughout 



r 




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a 

1 



s 

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5 .a 



tlS 



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I 



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CT5 
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f S 

E g 



no 

o 



134 Temperature 

the year. At Quito, Ecuador, for example, the average seasonal 
change is only 0.5C. 

Horizontal Changes. The changes in temperature from place to 
place over the surface of the globe run the whole gamut of extreme 
values that have been reviewed. Average temperatures on land are 
too variable for simple generalization, but a value of 32C may serve 
as an approximation for tropical regions and one of 12C for polar 
regions. For detailed information a textbook on climatology should 
be consulted, such as Kendrew (1949). In the sea average tempera- 
tures run from about 30C in the tropics to about 1.5C in the Arctic 
and Antarctic Oceans. 

Vertical Changes. The temperature of the air varies widely in a 
vertical direction according to local conditions, but a decrease of 
about 1C for every 150 m of altitude is generally found. On Mt. 
Washington in New Hampshire the tabulated average values were 
obtained for observations from 1933 to 1940. 

Feb. July Year 

Pinkham Notch (Altitude 610 m) - 8.5C 17.1C 4.2C 

Summit (Altitude 1915 m) -14.9 9.9 -2.8 



Difference: 6.4C 7.2C 7.0C 

The unequal heating of the air horizontally and vertically causes 
movements of the atmosphere. These are manifested as local winds, 
trade winds, and storms with consequent further influence on tem- 
perature, precipitation, and other ecological factors. 

Air is heated at all levels by the solar radiation that it absorbs and 
similarly may cool at all levels, but the greatest amount of heating 
and cooling takes place at the bottom. This effect is due to the great 
transparency of the air and the fact that the surface of the earth is 
heated faster during the day and cools by radiation more rapidly dur- 
ing the night than the atmosphere. The air in immediate contact with 
the earth therefore changes temperature more rapidly than strata at 
greater altitudes. The easy mobility of the atmosphere also contrib- 
utes to the thermal variations of the air environment. When air 
overlying heated land masses becomes warm, it tends to rise and to 
leave the earth's surface, but air chilled by contact with cold earth is 
heavier and tends to accumulate in hollows of the landscape. In 
these respects air stands in distinct contrast to the water environment 
in which heating and cooling take place principally at the upper 
surface. 

The spectral distribution of the energy received from the sun at the 



vertical U flanges 135 

earth's surface is shown in Fig. 5.3. It will be seen that about half of 
the solar radiation is in the infrared region and this represents a di- 
rect supply of heat. The visible and ultraviolet portions of the sun's 
emission will also produce a heating effect after they have become 
absorbed. The sample calculations in Table 7 illustrate the relative 
absorption of the various parts of the spectrum by air and water. 




3000 5000 7000 9000 11,000 13,000 15,000 17,000 19,000 A 
Wavelength 

Data from Fowle, 1927 

Fie. 5.3. Spectral distribution of solar energy at the earth's surface, showing the 
ultraviolet (UV), visible (divided into color components), and infrared portions. 

TABLE 7 

COMPARISON OF RELATIVE ABSORPTION OF SUN'S RADIATION BY AIR AND 

WATER 

Visible Light Infrared Radiation 

at 5500 A at 8000 A 

Percentage Percentage Percentage Percentage 

of initial absorbed of initial absorbed 
by each 
stratum 



Outside atmosphere 



value 

remaining 

100 



value by each 

remaining stratum 
100 



Mt. Whitney, California 93 

(summit 4420 m) 

Sea level 75 

2 m below surface 71 

30 m below surface 17 
(clearest ocean water) 



5 

70 



97 

88 

2 



9 

98 



136 Temperature 

The data show that the atmosphere absorbs radiation of short wave- 
length faster than that of long wavelength, whereas water is most 
transparent to radiation in the central part of the visible region. In 
all parts of the spectrum radiation is absorbed by water at far higher 
rates than by air, and the contrast is particularly great in the 
infrared. The absorption by a few meters of water is enormously 
greater than by several thousand meters of air. In general we can 
say that in comparison with the atmosphere the water medium is al- 
most opaque to the sun's radiation. 

Because of the differences in mobility and transparency of the 
two media we find that the vertical temperature changes in the water 
environment are controlled differently from those in the air. Radia- 
tion from the sun is the only important source of heat for natural 
bodies of water. Tests have shown that no significant exchange of 
heat takes place between the water and the mud of the bottom. Since 
solar radiation is absorbed in the upper strata of water, water bodies 
receive heat only at their upper surfaces. Cooling of water can take 
place through radiation, evaporation, or the melting of ice. All of 
these processes occur at the surface in natural water bodies. Gen- 
erally speaking, the seas and the inland waters gain and lose their 
heat primarily at the top. 

It is not surprising to find as a consequence of the foregoing that in 
deep bodies of water the major changes in temperature are limited 
to the surface strata. In some situations these changes are superim- 
posed on a deeper, permanent temperature structure. In the tem- 
perate and tropical regions of the sea a permanent thermal gradient 
has been produced between the mixed layer near the surface and the 
deep layer filling the bottom of ocean basins. The depth and extent 
of the zone of relatively rapid temperature change, known as the 
permanent thermocline layer, differ in the various parts of the ocean 
but an illustrative example is given in Fig. 5.4 for water in the great 
central eddy of the Atlantic Ocean. From roughly 2000 m down 
to the bottom the water is everywhere just about 3C. Since the 
average depth of the ocean is approximately 4000 m, there is ob- 
viously a very large amount of space in which the temperature is uni- 
form month in and month out, year in and year out. If you were a 
fish restricted to 3C, you could nonetheless travel over more than 
60 per cent of the globe without being exposed to a significantly dif- 
ferent temperature. 

Although essentially all the incident radiation is received at the 
surface of natural water bodies, we know that some heat is eventually 
transferred to deeper layers. How does the heat get there? Radia- 



Vertical Changes 137 

tion from one water layer to another is negligible. Conduction 
through quiet water is similarly very slow. If all the water in Lake 
Constance with a depth of 100 m were cooled to 0C and then the 
uppermost layer heated to 30C, more than 100 years would be re- 
quired for a measurable amount of heat to reach the bottom by radia- 
tion and conduction alone. It is obvious that heat can be transferred 



Temperature,C 
8 12 16 




Deep layer 



2000 L. 

Modified from Iselin, 1936 

FIG. 5.4. Permanent vertical temperature structure in the deep water of the 

eentral North Atlantic. The seasonal changes in upper layers are indicated; (A) 

April, (B) August, (C) December, (D) February. 

to lower levels only by the actual movement of the water, that is, by 
vertical circulation. 

We are further convinced of this conclusion when we examine situa- 
tions where vertical circulation has been prevented. When the tem- 
perature of the bottom water of a pond has been reduced to 4C and 



138 Temperature 

ice has formed across the surface, no vertical stirring by the wind can 
take place. Although the air temperature may go far below zero, 
heat can be lost from the deeper layers of the pond only very slowly 
because of the insulating action of the ice and the upper strata of 
water. This is the chief reason why most ponds do not freeze to the 
bottom during the winter. 

Stirring is also inhibited when a sharp density gradient has been 
developed between the upper strata and the deeper layers. Such a 
condition arises in situations in which the surface water is warmer or 
less saline, and hence lighter, than the deeper water. Under these 
circumstances the vertical stability of the water tends to resist any 
efforts on the part of the wind and waves to produce stirring, and heat 
exchanges are consequently limited to the surface. When no impor- 
tant density gradient exists, vertical stirring can become effective and 
heat will be carried down. This condition commonly occurs during 
the spring and during the autumn in waters of temperate regions, 
Under some circumstances the density of the surface stratum may be- 
come actually greater than the underlying layers resulting in a gravita- 
tional overturn of the water. 

Seasonal Changes in Vertical Temperature Structure. The typical 
temperature cycle in a lake during the course of a year is indicated 
diagrammatically in Fig. 5.5. Since the maximum density of fresh 
water occurs at 4C, any warmer or colder water will float on top of 
water of this temperature. If the underlying water has a temperature 
lower than 4C as it may in the early spring, or higher than 4 C C, as it 
may in the autumn, then a top-heavy condition will develop. 
Archimedian forces aided by strong winds will then bring about the 
spring overturn and the fall overturn. Winter stagnation occurs when 
the lake is ice covered and summer stratification is found after a dis- 
tinct density gradient has been produced by the warming of the sur- 
face layers. The layer of rapid vertical temperature change is termed 
the thermocline and in lakes is defined as having a thermal gradient 
of at least 1C per m. In lakes the wind-stirred and largely homog- 
enous water layer above the thermocline is known as the epilirnnion 
and the relatively stagnant water mass below the thermocline as the 
hypolimnion. The very considerable changes in temperature and the 
occurrence of stagnation and of circulation which they control exert 
a profound effect upon the biota of the lake as will be discussed in 
later sections. For further details on the thermal cycles of lakes the 
reader should consult a textbook of limnology such as Welch ( 1952 ) 
orRuttner (1953). 

A similar seasonal cycle of temperature occurs in the ocean but the 



Vertical Changes 139 

circumstances are somewhat different. For a fuller treatment of the 
subject reference should be made to a textbook of oceanography such 
as Sverdrup et al. ( 1942). In the open ocean the seasonal changes are 
superimposed upon the permanent thermal structure as relatively 
minor fluctuations (Fig. 5.4), but in coastal regions the water may be 



Winter 
stagnation 

Ice cover 



20 



-> Spring overturn 




Summer _ 
stratification 



-+~ Fall overturn 



20 



Epilimnion 



Thermocline 




Hypo- 
limnion 



22 

21 
9 





Fie. 5.5. Diagrammatic vertical section of a deep lake of the temperate region, 

showing the seasonal cycle of temperature (by permission from Limnology by 

Welch, 1952, McGraw-Hill Book Co.) 

sufficiently shallow for the seasonal effects to reach the bottom (Fig. 
5.6). In this type of situation the thorough stirring of the water 
during the winter produces a uniform temperature from top to bottom 
with a minimum value in February for the North Temperate Zone. As 
spring comes on, the larger supply of solar heat, often combined with 
lowered salinity due to increased run-off, causes the surface water to 
become lighter and tends to stabilize the upper layers. This effect is 
self-accelerating. By August the relatively thin stratum at the top has 
attained its maximum temperature. Below this layer a sharp ther- 



140 Temperature 

mocline leads to the deeper water which is still quite cold. With the 
onset of autumn solar radiation has become reduced and the winds 
have been stronger with the result that the surface waters are stirred 
downward, and the thermocline is shifted deeper and eventually de- 
stroyed. By November the whole water column has become mixed 
at an intermediate temperature and remains uniform as it cools to its 
minimum temperature during the winter. 

Temperature, C 
2 4 6 8 10 12 14 16 18 20 22 24 




Fie. 5.6. Seasonal changes in the vertical distribution of temperature in coastal 
water off New York, showing the positions of the thermocline ('/') (Modified from 

Clarke, 1940.) 



The radiation received from the sun reaches a maximum in June in 
the North Temperate Zone, but owing to the lag in the heating effect 
the highest temperatures on land ordinarily occur in July. At the 
surface of water the maximum heat of the season is not experienced 
until August, and in subsurface layers the thermal peak is even more 
delayed. If you imagine yourself a worm living in the bottom mud 
in the situation represented by Fig. 5,6, "summer" for you would not 
begin until November! In similar fashion the seasons are often com- 
pletely reversed in deep lakes. 

This review of the thermal conditions has shown that in the ocean 
and in lakes there is no temperature too high and no temperature too 
low for active life of some kind. Temperature changes are much 
less and much slower than in the terrestrial environment. Further- 
more, organisms can usually get out of excessively high or low tem- 
peratures by a short journey into deeper water. In these respects life 
in the water is much easier than life on land. The results of this 



Biological Action of Temperature 141 

situation over the ages has been that many oceanic plants and animals 
have become adapted to a relatively stable temperature. 

The disadvantage of this easy life in the water is that when an un- 
usual change in temperature occurs dire results may follow. Many 
instances are known in which slight temperature changes have caused 
mass mortality in the aquatic environment. A spectacular example 
occurred in 1925 off the coast of Peru (Murphy, 1926). In the spring 
of that year the cool Humboldt Current, which flows northward along 
the coast, apparently moved slightly offshore, allowing the warm 
countercurrent, El Nino, to flow farther south than usual, raising the 
temperature 5 or 6C above normal. The result was that the plankton 
and a great many of the local fishes were killed. Larger fishes, de- 
pendent upon these smaller forms of life for food, soon afterward 
succumbed and washed up on the shore in windrows. In addition 
huge numbers of fish-eating birds died of starvation. Many of these 
birds inhabited coastal islands where their droppings produced guano 
deposits of extreme value as fertilizer. Guano production was 
stopped, and the coast line for miles was strewn with the carcasses of 
the dead birds. Torrential rains caused by the slight change in ocean 
temperature washed away tons of guano that had required generations 
to accumulate. The heavy rainfall also devastated the neighboring 
ordinarily arid land areas, disrupting the natural fauna and flora, and 
destroying crops, roads, and buildings. The ecological and economic 
repercussions of the oceanic change thus extended to the shore and 
inland. A few months later the ocean currents returned to their nor- 
mal course, and after a period of years the fauna and flora were grad- 
ually restored. This cataclysmic destruction of ocean life off Peru 
well illustrates the far-reaching consequences of a slight temperature 
change in a situation where the plants and animals have become ad- 
justed to relatively uniform conditions. 

BIOLOGICAL ACTION OF TEMPERATURE 

What are the ecological consequences on land and in the water of 
the thermal conditions that have just been reviewed? Environmental 
temperature exerts a direct action on the tissues of poikilothermous 
organisms, that is, on those whose body temperatures vary with the 
surroundings. The heat exchanges of homoiothermous animals are 
also affected by external thermal conditions, but the insides of these 
animals are thermostatically maintained at a nearly constant, and 
relatively high, temperature. 



142 Temperature 



Extreme Temperatures 

Minimum Temperatures. We may consider first the danger to the 
organism of the freezing, or the actual congealing, of its living tissues. 
Freezing is likely to produce mechanical harm in the form of the rup- 
ture of cell walls and the stoppage of circulation. When ice crystals 
are formed, they also withdraw water from neighboring areas, pro- 
ducing a condition similar to desiccation in the surrounding cells. No 
tissue will freeze at 0C. The concentration of solutes in body fluids 
and tissue fluids causes a depression of the freezing point with the 
result that even unprotected protoplasm will not freeze until a tem- 
perature a few degrees below zero is reached. For this reason many 
tissues can be active at 0C; rye seeds, for example, will germinate on 
ice. 

In situations where the temperature drops considerably below zero 
the danger of freezing is ever present, Some forms have special 
adaptations for lowering the freezing point of their tissues still farther 
so that congealing does not take place until a very low temperature is 
reached. In addition a few species are able to withstand actual freez- 
ing for short periods of time. The green alga, Chlorella, while in 
active vegetative condition, has been frozen at 182C for 1 hour 
without harm. At subzero temperatures ice crystals are known to 
occur in the needles and wood of the Norway spruce, and their pres- 
ence evidently produces no serious injury to the tree. Among the 
higher animals the Alaskan blackfish has the ability to recover normal 
activity after being frozen for periods of 40 minutes at temperatures 
down to 20C. 

Many plants and animals are killed by temperatures that are too low 
for them but are nevertheless far above values at which tissues would 
actually freeze. Similarly, at the other end of the scale we find many 
instances of death from excessively high temperatures but at values 
below those producing heat coagulation. Lethal extremes vary greatly 
from species to species, and the organism as a whole may be killed by 
a degree of chilling insufficient to cause direct damage to individual 
protoplasmic structures. Temperatures that are too low for some 
species may be favorable, or even too high, for others. The English 
daisy grows and flowers best when subjected to night temperatures 
below 10C and dies if kept for long periods above 20C. In contrast 
the African violet is killed by long exposure to night temperatures 
below 10C and grows best and flowers when the thermometer stands 



Maximum Temperatures 143 

above 20C (Went, 1950). Many Arctic fish do not venture south 
into waters as warm as 10C, but this same temperature would be far 
too cold for most tropical fish. 

If the tissues do not freeze, why are these animals and plants killed 
by moderately low temperatures? Most chemical reactions are slowed 
down by lowering temperature and eventually stop. The cessation 
of any one of the vital processes will cause the death of the organism. 
However, the various processes going on in the body come to a stop 
at various points on the temperature scale. No one biological zero 
exists for all reactions even within the same individual. Under some 
circumstances the organism may die when all the vital processes are 
still going. The explanation may be that the different reactions are 
slowed down by different amounts with the result that mutually de- 
pendent processes get out of adjustment and cause the death of the 
organism. 

The minimum temperature for the organism as a whole is thus de- 
termined by the most susceptible of the vital processes. The lowest 
temperature at which the organism can live indefinitely in an active 
state is termed the minimum effective temperature. After a further 
reduction of temperature the organism goes into chill coma. If too 
long a period does not elapse before the organism is again warmed, it 
will become active once more. The lowest temperature at which 
survival is possible is called the minimum survival temperature. The 
actual value of this temperature depends upon the period of exposure. 
For example, the eggs and larvae of the fruit fly, Ceratitis capitata, 
were killed at 7C only after an exposure of 7 weeks, but death was 
caused after 3 weeks at 4C or after 2 weeks at 1C. Accordingly, a 
statement of the minimum survival temperature for an animal or a 
plant has no meaning unless the period of exposure is also stated. 

Maximum Temperatures. At the other end of the temperature 
scale the same sort of situation exists. The maximum effective tem- 
perature is the greatest intensity of heat at which the species can live 
indefinitely in the active state. The effective temperature range within 
which the organism can carry on its active life and beyond which death 
eventually results extends between the maximum and minimum effec- 
tive temperatures. At higher temperatures the organism goes into 
heat coma but will recover if restored before too long to cooler con- 
ditions. For the maximum survival temperature the period of the ex- 
posure of the organism to the specific heat conditions must be given. 
All these thermal relations are illustrated for the house fly in Table 8. 



144 Temperature 



Methods of Meeting Temperature Extremes 

A surprising number of plants and animals can withstand consider- 
able thermal fluctuation but do not possess any special mechanism for 
meeting the problem of temperature extremes. When the ther- 
mometer rises too high or drops too low, these animals and plants 
must either "take it" or die. This is the situation for most sessile 
aquat|c invertebrates, for many other invertebrates, for many plants, 
and for some higher animals. As we have seen, getting along without a 
special temperature adaptation is easily possible for many organisms in 
the aquatic environment. On land, however, north and south of the 
killing frost line, and in areas exposed to intense heat, something must 
be done to survive periods when the temperature exceeds the effec- 
tive range within which active life can be maintained. 

Morphological and Physiological Adaptations. During the course 
of evolution many specialized structures have been developed that 
relate to the temperature problem. Certain animals and plants pro- 
duce spores, cysts, eggs, pupae, or seeds that are capable of resisting 
thermal extremes. Each of these structures represents a whole organ- 
ism. In other species special parts of the fully developed organism 
are resistant to extremes of heat and cold. Such is the situation with 
the hardy roots and stems of many perennial plants. For example, 
the tops of grass plants freeze off in winter. Life remains in the 
stolons and roots of the grass, and from these organs new leaves are 
produced when warm weather again returns. 

TABLE 8 
TEMPERATURE RELATIONS OF THE HOUSE FLY, Musca domestica 

Death 4fl.5C in few minutes Maximum survival temperature 

Heat coma 44 . 6 

Excess! ve acti vi ty 40.1 

Rapid movement 27.9 \ Maximum effective temperature 

{23 I 

> Effective temperature range 
lo I 

Feeble movement 10.8 / Minimum effective temperature 

Begins moving 6.7 

Chill coma 6 . 

Death 5.0 in 40 minutes Minimum survival temperature 

8.0 in 20 minutes Minimum survival temperature 

12.0 in 5 minutes Minimum survival temperature 

In some species physiological changes take place in the tissues that 
prevent freezing. Osmotic concentration is increased, and water is 



Morphological and Physiological Adaptations 145 

"bound" in colloid form with the result that the freezing point is de- 
pressed below values ordinarily experienced. If you walk through a 
field of winter rye on a day when the temperature is well below 0C, 
you will notice that the leaves and stems are flexible and not brittle as 
they would be if the tissues were frozen. The completion of these 
hardening processes takes time, and the actual high or low tempera- 
tures that an animal or a plant can withstand often depend upon the 
interval available for acclimatization. Evergreen trees and many 
other types of plants similarly become "frost hardy" during the winter. 
Everyone is familiar with the ability of crocuses, snowdrops, and other 
spring flowers to withstand frost and even to push up through a layer 
of snow. On one occasion a fully opened crocus blossom was ob- 
served unharmed after a night during which the air temperature had 
dropped to 16C. Various species of alpine plants exhibit a se- 
quence in their flowering in or near snowbanks according to their 
differing tolerance of low temperatures. Many insects are similarly 
able to form bound water in their tissues and thus avoid the dangers 
of freezing. 

Another method by which extremes of temperature are endured 
involves the removal of water from the tissues. Dried seeds, spores, 
and cysts avoid freezing because no liquid remains that can freeze. 
Dry seeds have germinated successfully even after exposure for 3 
weeks to liquid air (about 190C). In another experiment nema- 
todes and tardigrades are reported to have recovered after chilling for 
several hours at an even lower temperature. At the other extreme 
tardigrades in a resistant condition have endured immersion in boiling 
water for short periods of time, and dried cysts of the ciliate, Colpoda 
cuculluSy have survived after exposure to dry heat at 100C for 3 days. 
The even greater thermal resistance of some bacterial cysts is well 
known. Among some plants unusually high temperatures not only 
are tolerated but also actually accelerate development. Seeds of the 
wattlebark tree are commonly boiled 1 hour to hasten germination 
before planting. The seed cones of certain species of pine trees (e.g., 
Pinus banksiana] open promptly only after fire has scorched them. 
This fact has obvious implications for the ecological effects of forest 
fires as will be discussed later. 

Another special adaptation for dealing with extremes of temperature 
is dormancy. The term hibernation is often used loosely to describe 
all instances in which metabolism is reduced during winter when the 
environment becomes too cold. A great many poikilothermous ani- 
mals go into such a hibernating condition in crevices, under rocks, or 
in the mud. Plants similarly are sometimes spoken of as hibernating 



146 



Temperature 



when in the dormant winter condition. Plant dormancy is induced 
by lowered autumn temperatures often accompanied by desiccation. 
Dormancy during the summer, when high temperatures, excessive 
dryness, and/or shortage of food may occur, is called aestivation, and, 
although less common than hibernation, it is found widely among 
insects and some other invertebrates, as well as among plants and 
certain mammals. In many insects dormancy takes the form of a 
diapause, that is, a stage in the development of the animal during 
which morphological growth and development are suspended or 
greatly retarded. Species having the capacity for diapause usually 
display a rhythm in the life cycle that is related to the seasons. 
During periods of unfavorable climate the greater part of the popu- 
lation is in the resistant, diapause stage. Anclrewartha (1952) re- 
ports that in the absence of diapause the grasshopper, Austroicetes 
cruciata, which maintains high numbers over a wide area in southern 
Australia, would almost certainly die out in most of this area, or at 
best become a very rare species. Since in general dormancy is cor- 
related with severe environmental conditions, insects of the same 



LOCALITY 


MONTHS 

I II HI IV V VI VII VIII IX X XI XII 


London 
Berlin 
Paris 
Nice 
Naples 
Athens 
Ankara 
Tel aviv 
Cairo 
Khartoum 
Leningrad 
Tiflis 
Moscow 
Formosa 




































1 


























1 






















1 




2 


















1 


,2 






2 














1 












1 














1 








1 












1 




, 2 










2 


3 










1 


2 










2, 






1 


, 2 




















2 






































1 




















1 


,.... 
















1 




2 










2 





Alter Bodenheirner, 1938 

FIG. 5.7. Variation in the seasons of dormancy and in the number of complete 
and partial generations in the ladybird beetle, Coccinella septempunctata, in 

different parts of its range. development; aestivation; 

hibernation. 



Morphological and Physiological Adaptations 147 

species in different parts of the world may go into hibernation or 
aestivation at quite different periods of the year (Fig. 5.7). 

Certain mammals, and at least one bird (Jaeger, 1949), also become 
dormant with reduced metabolism under seasonal extremes of climate. 
Since this type of dormancy involves a physiological change peculiar 
to warm-blooded animals, a separate term should be employed to 
describe it, but in the absence of a special word the terms hibernation 
and aestivation are used for mammals (and birds) as well as for 
lower animals (Lyman and Chatfield, 1950). In mammalian hiber- 
nation the internal temperature drops to about 1 degree above the 
temperature of the animal's surroundings, provided that the latter does 
not fall lower than a few degrees above 0C. This is not likely to 
happen in the burrows or caves where the animals usually hibernate 
(Fig. 5.8). In laboratory experiments a reduction of the external 




Photo by D. R. Griffin 

FIG, 5.8. Hibernating bats, Myotis I. lucifugus, hanging in clusters head down- 
ward by hind claws hooked to the rough limestone in a Vermont cave. 

temperature below 0C causes the hibernating mammal either to be 
aroused from its dormant condition or to be killed by freezing. 
Temperatures well below 0C for long periods in the winter would 
prevent any mammal from hibernating in the Arctic unless it could 
find a sufficiently tempered retreat. Whether for this or other reasons 
there are few hibernating mammals in the far north. The Arctic 



148 Temperature 

ground squirrel hibernates in burrows made in local areas of unfrozen 
ground under deep snow. Other small mammals remain active in 
lined nests made in the snow itself. Measurements at Fairbanks, 
Alaska, showed that at less than 1 m beneath the snow surface a micro- 
climate temperature of 5C existed at a time when a value of 
50C was recorded in the air just above the snow. 

Less is known about the physiological adjustments of aestivation 
in mammals, but this type of dormancy similarly involves a shift of 
metabolism into low gear and a complete change in the control of 
internal temperature (Hamilton, 1939). An example of mammalian 
aestivation is found in the summer dormancy of the ground squirrels 
of southern California. By remaining torpid in their burrows for 
several months these animals not only avoid the high summer tem- 
peratures but also tide over periods of water and food shortage. 

A special device for dealing with extremes of temperature while in 
the active condition is homoiothermy, or warm-bloodedness. Birds 
and mammals are able to maintain a remarkably constant internal 
temperature despite great variations in the outside world. Limits 
exist, of course, beyond which these animals cannot maintain their 
temperature control, but by allowing a regulated amount of evapora- 
tion to take place from their bodies they can keep their own tempera- 
tures down to normal values under the highest environmental tem- 
peratures ordinarily encountered. Under extremes of cold weather 
birds and mammals are able to maintain their relatively high internal 
temperature through the insulating action of fur, feathers, and fat, as 
well as by suitable physiological adjustments (Scholander et al., 1953). 
The remarkable ability of mammals to maintain their normal thermal 
level under severe winter conditions is exemplified by a varying hare 
whose internal temperature was found to be at its normal value of 
38C on a day when the thermometer stood at 46C. 

Thermal Migrations. Another method for dealing with excessively 
high or low temperature conditions is to move out of them. This 
method is obviously available only for locomotory forms, and it cannot 
be used by the majority of plants. Journeys taken by animals that 
enable them to escape from extremely hot or cold situations are referred 
to as thermal migrations. Some of these migrations are relatively 
short trips involving movements of only a few meters or even a few 
centimeters. Thermal migrations are made on a small scale from ex- 
posed positions to the shade to avoid the scorching heat of the desert 
and from shade to sun in cold regions. We commonly think of desert 
animals as being able to withstand extremely high temperatures. As 
a matter of fact most desert animals have become nocturnal in their 



Thermal Migrations 149 

habits and thus avoid the heat of the day. This is true even of char- 
acteristic desert reptiles such as the rattlesnake. Ecologists working 
in the desert areas of southern California have found that rattlesnakes 
will succumb if forced to remain for more than 15 minutes on the hot 
desert soil exposed to the midday sun. The red racer is one of the 
few desert reptiles that venture forth regularly in the daytime. Since 
this species is one of the fleetest of the snakes, it is able to cross hot 
areas and get back into the shade before becoming harmfully heated. 

In less severe environments we are familiar with the short journeys 
of terrestrial amphibians into shaded places in hot weather. How- 
ever, the chief benefit from a movement out of the sun for salamanders 
and other forms with wet, permeable skins may be conserving water 
rather than keeping cool. The excessive evaporation experienced by 
an amphibian in an exposed, windy location would cause a chilling of 
the body, and, in addition to avoiding water loss, the animal might go 
into a sheltered place to keep warm. 

The short trips into or out of the water made by frogs, turtles, and 
other amphibious forms are familiar to everyone and serve to provide 
cooling or wanning for the animals concerned. An unusual tempera- 
ture relation is displayed by the white pelicans that nest on islands of 
the Salton Sea, California. In this extremely hot location the eggs 
and young in the nest would be killed if exposed for more than about 
20 minutes to the intense radiation from the sun. The brooding of 
the mother birds acts to keep the eggs and young cool rather than to 
keep them warm! At intervals the adult pelicans wet their plumage 
by a trip into the water and take advantage of the cooling produced 
by evaporation. 

Burrowing animals escape excessive heat or cold by short journeys 
deeper into their substrata. Soil organisms avoid summer heat by 
moving deeper into the earth. Ground squirrels are included in the 
desert fauna of southwestern United States but they are rarely exposed 
to the extremes of heat in that area. In the hot season these rodents 
retire into their burrows where a microclimate of much more mod- 
erate temperatures prevails (Table 9). At a later season the same 

TABLE 9 

REDUCTION IN TEMPERATURE AT INCREASING DEPTHS HENEATH THE SURFACE 
OF THE DESERT AT TUCSON, ARIZONA 

Air (maximum) 42.5C 

Surface (maximum) 71.5 

10 cm below surface 41.1 

30 cm below surface 29 . 8 

45 cm below surface 27.9 



150 Temperature 

type of migration serves to avoid the severity of winter. Some ani- 
mals work their way into rotten logs, others dig deep in the soil, and 
many species squirm into the mud of swamps or pools. This move- 
ment of animals into winter quarters is often loosely referred to as 
hibernation, and should not be confused with the structural and 
physiological changes often occurring in winter dormancy and de- 
scribed by the same term. 

Somewhat longer journeys that result either primarily or secondarily 
in the avoidance of extreme temperatures are made by larger land ani- 
mals. Bear, deer, and other game animals descend from the moun- 
tains into the sheltered valleys when the weather gets cold. In the 
spring they return to higher elevations and to more exposed situations. 
The same type of thermal migration of moderate length is seen in the 
aquatic environment. Many fishes and other active aquatic animals 
leave the shore in the summer when the water has become too warm. 
Conversely, other species migrate into deeper water during the 
winter to avoid what are for them excessively low temperatures. 
Such migrations at contrasting seasons are illustrated very neatly by 
two species of flounder that inhabit the coastal areas of New England. 
The so-called summer flounder (Paralichthys dentatus) enters the 
shallow water of the bays in June and departs in October. The winter 
flounder (Pseudopleuronectes arnericanus), on the other hand, is not 
able to tolerate the inshore temperatures characteristic of July and 
August. Fish of the latter species are abundant in the upper reaches 
of Great South Bay, Long Island, during the winter. As spring comes 
on the population moves toward the mouth of the bay, and, when the 
summer sun warms the water beyond about 20C, the majority of this 
species migrates to the cooler, deeper water outside the inlet (Fig. 
5.9). During the autumn months the winter flounder returns once 
more into the bay. 

Migrations of still greater length are carried out by certain mam- 
mals, insects, and birds. Although change of temperature is not the 
primary cause of some of these mass movements, escape from climatic 
extremes is certainly an important consequence in many instances. 
The caribou migrate long distances, and in former times gigantic herds 
of bison traveled hundreds of miles across the American plains from 
their winter grounds to their summer feeding areas. The extensive 
north-south migrations exhibited by some insect species should also 
be considered as thermal migrations in part at least. Most spectacular 
of all are the trips made by migrating birds. Considerable evidence 
has now accumulated to indicate that change in day length is the 
factor that initiates the southward migration of birds at the end of the 



Thermal Migrations 



151 



breeding season. However, many species of birds inhabiting high 
latitudes during the summer would succumb from temperature alone 
if they remained in the far north through the winter. Other species 
could perhaps withstand the winter cold but would succumb through 
an indirect action of the heat factor resulting from the disappearance 
of their food supply or from the inability of the birds to obtain the 
larger amount of food necessary to maintain their internal tempera- 
ture in winter. For these reasons it seems probable that the avoid- 





Mar. Apr, 




12' 



May June 




July Aug. Sept. 




Modified from Neville and Perlmulter, 1941 

FIG. 5.9. Thermal migration of the winter flounder as revealed by the recapture, 
at the points indicated, of fish tagged and released in Great South Bay, Long 

Island. 



152 Temperature 

ance of temperature extremes played an important role in the evolu- 
tionary development of the migration habit of birds and of other ani- 
mals, even if this factor does not provide the trigger that sets off the 
seasonal journeys each year. 

Action within Effective Range 

Although the action of temperature extremes may be drastic at 
times, the plants and animals of any habitat spend most of their lives 
at intermediate temperatures. We shall now consider the action of 
the heat factor in the range between the minimum effective tempera- 
ture and the maximum effective temperature. Just as with extremes 
of heat and cold, the influence of a given temperature within the effec- 
tive range depends upon the thermal conditions to which the species 
of plant or animal is adapted. Many tropical, temperate, and polar 
species have become attuned to the temperatures characterizing their 
respective regions so that many of their life processes go forward at 
approximately the same rate in spite of considerable temperature dif- 
ferences. For example, the rate of oxygen consumption in certain 
lamellibranchs is of the same order of magnitude for species living at 
0C in the Arctic, at 8C in boreal seas, at 12C in the Mediter- 
ranean, and at 27C in tropical waters (Thorson, 1950). Seasonal 
adjustment to temperature is exhibited by the sand crab (Ernerita 
talpoida) in the Woods Hole area to the extent that its metabolism 
at 3C in winter is four times greater than in summer. As a result 
the animal can be active and can grow during the winter when many 
other species in the tidal zone become inactive (Edwards and Irving, 
1943). The action of specific temperatures on life processes must 
be considered for each species separately, and also for geographical 
subspecies, but we can delineate certain generalities in regard to the 
role of the heat factor within the effective range. 

Effect of Temperature on Biological Rates. The influence of tem- 
perature on the rates of biological processes is most clearly seen in 
poikilothermous animals and plants. Although mammals and birds 
are definitely affected by the heat factor, the action is largely indirect 
because these warm-blooded forms carry their own temperature 
around with them. When acting directly, a rise in temperature of 
10C usually causes a doubling or a tripling of the rate of a biological 
process, and the process is said to have a Q 10 of 2 or 3 in accordance 
with Van't Hoff's law. 

If you will take the trouble to listen to the cadence of the crickets on 
several evenings, you will observe that the frequency of their chirping 



Effect of Temperature on Biological Rates 



153 



is higher in warm weather and lower in cool weather (Fig. 5.10). 
This relationship was put to use by a blind astronomer who was able 
to "read' the temperature by timing the chirps produced by the crickets 
outside his house and applying a formula that he had worked out 
(number of chirps in 13 sec -f- 42 rr F). 

Another example of the increase in reaction speed caused by rise in 
temperature is given in Table 10. The data show that the cod egg will 
develop at a temperature as low as 1C. Its speed of development 
increases regularly to a temperature of 14C, above which the egg 
will no longer survive. The relationship with temperature in this 



30 



.20 




10 




100 



220 



f 
e 

300 



200.2 

g a. 



140 180 

Stimulations per minute 

Fie. 5.10. Relation of stridulution rate of tree cricket Oecanthus to temperature 
and also to pitch. The note C is "middle C." (Modified from Matthews, 1942, 

Science. ) 



TABLE 10 

RELATION BETWEEN TEMPEKATIWE AND RATE OF DEVELOPMENT OF 
OF A C()LI)-WATEIi AM) A \VAKM-WATKK FlSH 



Cod 

2C No development (ecological zero) 

I Hatch in 42 davs 



o 

8 

10 

12 

14 



18 
13 

10.5 
f) 7 
8.. r > 
No development 



Mackerel 
3C No development 



10 

15 

18 
20 



Hatch in 207 hours 
150 
10,5 
70 

50 
No development 



154 Temperature 

and in most other biological processes is not linear, and sometimes 
varies in a complicated way. For the cod egg a rise of 4 from 
1C to -f* 3C produces a doubling in the rate of development, but 
a similar increase of 4 from 10C to 14C results in only a slight 
acceleration. A temperature of 10C, which is the minimum at which 
the mackerel egg will develop, is near the maximum temperature for 
the cod egg. As the temperature is raised from 10C to 21C the rate 
of development of the mackerel egg is increased until at the upper 
value it will hatch in just over 2 days. 

An increase in temperature beyond a critical point may cause a 
reduction in the rate of some life processes in certain species. The 
growth of the hypocotyl of the pea seedling shown in Table 11 illus- 

TABLK 11 

RELATIOX BETWEEN TEMPERATURE AND THE GROWTH OF THE PEA SEEDLING 
As INDICATED in THE ELONGATION OF THE HYPOCOTYL 

Temperature Growth 

C Mm per Day 

14.1 5 

18.0 8 

23 . 5 30 

"2(> 6 54 

28 . 5 40 

33 ,5 23 

3(i . 5 1) 

trates this point. As temperature increases from 14.1C to 26.6C 
the amount of growtli recorded each day steadily increases, but at 
higher values the daily elongation of the hypocotyl becomes less. In 
contrast to the growth in fish eggs, the pea seedling continues to 
elongate after the temperature has been reached at which growth is 
the fastest. This fact does not moan that increase in temperature has 
a negative effect on a reaction. Growth, like most other biological 
processes, is the result of a number of biochemical reactions going on 
simultaneously. In the present instance the rate of some inhibiting 
reaction may have been increased more by a rise in temperature than 
the reaction for elongation with the result that after a certain point 
the rate of growth becomes less. 

In other instances temperature may have little or no effect on the 
rate of biological processes. Between certain limits the incubation 
period for many reptiles is only slightly influenced by thermal changes 
in the surroundings. The diamond-backed terrapin, for example, 
undergoes normal hatching between 18C and 33C, but within that 



Optimum Temperature 155 

range no regular correlation exists between temperature and time of 
development. On the other hand, some life processes go forward 
only within a very narrow thermal range. Everyone who has raised 
chickens knows that it is impossible to persuade the eggs to hatch 
more quickly by increasing the temperature. Although the hen's egg 
has no temperature control of its own, it will hatch only if maintained 
between 40C and 41C and only after 21 days of incubation. 

Optimum Temperature. The foregoing discussion of the influence 
of temperature on the rates of biological processes leads to a con- 
sideration of the optimum temperature. However, the concept of 
the optimum in ecology is a slippery customer unless the life process 
concerned is specified. The optimum temperature may be considered 
that value at which a certain process goes on the fastest. But the tem- 
perature for the maximum rate often varies considerably for different 
processes within the same organism and also for the same process at 
different stages in the life cycle. 

Among plants in the temperate zone the optimum temperature for 
germination usually differs markedly from the optimum value for the 
fruiting process, and the optimum heat condition for photosynthesis 
may occur at a value different from either of the other two. Similar 
variations in thermal relations are found among animals. The optima 
for the various developmental stages are frequently found at values 
widely different from the optimum for the adult. The eggs and larvae 
of terrestrial species usually require a higher temperature than the 
adult stage, but notable exceptions are provided by many animals as, 
for example, by the corn borer, and the eastern brook trout. The trout 
spawns in October or November, and the eggs can develop at 4C or 
less. The optimum temperature for the development of trout eggs is 
8C. The adult trout, on the other hand, do not feed much and there- 
fore do not grow until the water is warmer than 10C. The optimum 
for the growth of the adult falls between 13C and 16C. These ex- 
amples are sufficient to indicate that no one point on the thermal scale 
can be designated as the optimum for the entire growth of an organism, 
but that optimal values differ according to the life stage of each 
species. 

The most favorable temperature for survival often differs consider- 
ably from the optimum value for growth, reproduction, and other 
life processes. Mackerel eggs develop at the fastest rate at 21C, but 
in the same investigation the survival of the eggs was found to be best 
at 15C. Other experiments showed that the life span of the common 
water flea, Daphnia magna, is greatest at 8C (Table 12). In con- 
trast, the shortest time for the production of the first batch of young 



156 Temperature 

Daphnia occurs at 23C, and the optimum from the point of view of 
the largest number of young produced is found to be at 18C (Mac- 
Arthur and Baillie, 1929). The ecological effect of these divergent 
optimal values will depend upon whether they are applied to the 
individual or to the population. The individual Daphnia lives longest 
at one temperature, but the population as a whole grows the fastest 
at another temperature. The optimum temperature for a species can 
therefore be only a general concept. The optimum is best thought of 
as a range of temperatures, and it is the range within which the or- 
ganism as a whole functions best. 

TABLE 12 

TEMPERATURE RELATIONS OF THE CLADOCERAN, Daphnia magna 

Average Length Time for Production Average Number of 

of Life (days) of First Young (days) Young Produced 

8C 108 

10 88 

1* 16 6 

18 40 8 24 

23 6 9 

28 26 

Other Effects of Temperature. In our discussion of temperature 
thus far we have dealt chiefly with growth and survival. Many other 
influences of this factor are of importance in the ecology of animals 
and plants. A drop in temperature ordinarily means a decrease in 
activity, especially among the cold-blooded animals and plants. As 
winter comes on, insects and other invertebrates ordinarily become 
dormant. In the spring on land the thermometer usually must climb 
above 8C before cold-blooded animals become abundantly active. 
The snow flea, Collembola, forms an interesting exception to the fore- 
going generality. These small black insects are often found swarming 
on the surface of the snow among the trees in February or March. 
Evidently the animals can absorb enough heat from the strengthening 
spring sun in their protected microclimates at the base of trees to be 
active. In the aquatic environment many animals are adapted to 
carrying on a very active life at temperatures only a few degrees 
above 0C. No one who has hooked a salmon in a northern stream 
will claim that a high degree of activity is impossible for cold-blooded 
animals at these lower temperatures. 

Certain behavior patterns are influenced by the heat factor. Some 
of the more primitive animals exhibit a thermptaxis, that is, an orienta- 
tion toward a source of heat Ticks are aided in locating their warm- 



Other Effects of Temperature 157 

blooded hosts by a turning reaction to the heat of their bodies. Rat- 
tlesnakes, copperheads, and other "pit vipers" can detect the presence 
of mammals or birds that may be only a few degrees warmer than 
the surroundings. Oriented by the heat radiation, these snakes can 
strike accurately at their prey even in the dark. Other behavior 
reactions are influenced by heat conditions. When the temperature 
drops below 5C, the leopard frog is stimulated to burrow into the 
mud bottom of its pond. At the same temperature sunfish crowd 
together in aggregations but, if the water is warmed to 8C, the fish 
tend to swim about separately. Snakes similarly gather in aggrega- 
tions and form balls in their retreats among the rocks when cold 
weather arrives in the autumn. 

Temperature, as well as moisture, light, and other factors, has long 
been known to affect the coloration of some animals. In warm, humid 
climates many mammals, birds, and insects, tend to be more mclanic, 
that is, darker in color than races of the same species living in cool, 
dry climates. This generality, already referred to as Gloger's rule, is 
well recognized but many exceptions are encountered, and the relative 
influences of heat and moisture are not known. In some instances 
coloration seems more affected by genetic selection than by the direct 
action of climate. The possibility of hereditary transmission of heat- 
color relationships is demonstrated by experiments showing that the 
temperature to which pupae are exposed will affect the wing color of 
butterflies even to the second generation. This whole subject is con- 
troversial since observations on various species in different areas do 
not agree. Coloration is evidently controlled by interactions between 
environmental and hereditary factors, but a more exact understanding 
must await further study. 

The temperature factor is also known to affect the absolute size 
of many animals as well as the relative proportions of certain parts. 
The general fact that among birds and mammals the same species 
attains a greater body size in cold regions than in warm regions and 
that among closely related species the larger ones inhabit the colder 
climates is known as the Bergtnann principle. Poikilothermous ani- 
mals, as exemplified particularly by reptiles and amphibians, exhibit 
the reverse relationship since they tend to be smaller in colder cli- 
mates. 

The related observation that extremities, such as the tail, ears, and 
legs, of mammals are shorter in colder climates has been delineated 
as Aliens rule. Since both these generalities apply to warm-blooded 
forms, they are probably related to the difficulty of retaining heat at 
low temperatures and the desirability of losing it at excessively high 



158 Temperature 

temperatures. Animals with large bodies have surfaces that are rela- 
tively smaller in relation to their masses than small animals. Smaller 
extremities expose smaller surfaces (Fig. 5.11). Since heat is lost 
through the surface, the smaller the area of an animal's skin, the more 
easily may the animal maintain its temperature in cold weather. 
Conversely, the development of extremities with large areas aids heat 
loss and evaporation in hot climates. These simple ecological rela- 
tionships, in addition to underlying Bergmann's principle and Allen's 
rule, undoubtedly account in part at least for the fact that no extremely 
small mammals or birds exist, that is, as small as the majority of insects. 




Fie. 5.11. Head of (left) arctic fox, Alopex lagopus, (center) red fox, Vulpes 

vulpes, and (right) desert fox, Megalotis zerda, showing gradation in size of ears. 

(Reprinted with permission from Hesse, Allee, and Schmidt, Ecological Animal 

Geography, 1951, John Wiley & Sons, New York.) 



A very specialized influence of temperature on morphology is the 
apparent control it exerts on the number of vertebrae in certain species 
of fish a relationship known as Jordan's rule. Cod hatched off New- 
foundland where temperature ranges between 4 and 8C have 56 
vertebrae, whereas cod hatched east of Nantucket in temperatures 
averaging 10 to 11C possess only 54 vertebrae. The relationship 
is brought about through the control by temperature of metameric 
segmentation at an early stage of development. Incidentally, the 
fact is of use to fishery biologists in ascertaining the origin of popula- 
tions among those species of fish that exhibit this geographical dif- 
ference in structure. 

The temperature of the environment controls egg type and sex 
ratio in certain animals. Under moderate heat conditions Cladocera 
produce parthenogenetic eggs, and these hatch usually into females. 
Under ordinary circumstances few if any males exist in the popula- 
tion, but their production and the appearance of "winter" eggs is 
favored by extremes of temperature. The winter egg of the Clado- 



Mode of Temperature Limitation 159 

cera, as described earlier, is able to withstand desiccation and freez- 
ing but, since it is a sexual egg, it must be fertilized by a male before 
it can develop (cf. Fig. 2,11). In Moina macrocopa, when the tem- 
perature drops below about 14C, or rises above 30C, males appear in 
the population and sexual eggs are produced. This control of egg type 
and sex ratio is attuned to the fact that in temporary pools the oc- 
currence of very high temperatures often precedes the drying up of 
the pond in summer. At the other end of the year a drop in tempera- 
ture will presage the possibility of freezing and the necessity that the 
animal go into a life structure in which the severity of winter can be 
endured. 

ACTION OF TEMPERATURE ON DISTRIBUTION 

Mode of Temperature Limitation 

In many early attempts to describe the ecological action of tem- 
perature an effort was made to correlate geographical distribution 
with mean temperatures. Although we see many average tempera- 
tures in print, we never encounter one in nature. A distinction must 
be made between the use of a temperature value as an index and the 
action of temperature as an agent controlling distribution. The limits 
to the spread of a population are always set by ecological factors act- 
ing on the individuals making up the population. In most situations 
the individual is not subject to a temperature equal to the average 
temperature of the region for more than a few hours of the day or 
a few days of the year. 

In eastern Massachusetts the mean annual temperature is about 
7C but on many days in summer the thermometer stands near 24C 
and in the winter near 10C with extreme records far above and 
below these values. As a consequence the fauna and flora of the 
region are very different from what they would be if the temperature 
remained in the neighborhood of 7C all the year round. In the 
latter event the local biota would be more like that of the Aleutian 
Islands or of South Iceland where the mean monthly temperature 
varies only from about 10 to 1C with an average of about 4C. 
We may conclude that the mean annual temperature is not the chief 
aspect of this factor which controls distribution, but a more pene- 
trating analysis of the action of temperature must be made. 

Careful consideration of the various thermal influences shows that 
for the permanent life of a species in an area the following tempera- 
ture requirements must be met: 



160 Temperature 

a. The temperature must never be so high or so low at any time as 
to kill the organism. 

b. The temperature must be high enough, or in some instances low 
enough, for a sufficient period to permit the reproduction and 
growth of the species. 

Limiting minimal temperatures for survival ordinarily occur in 
winter, and minimal heat required for reproduction and growth will 
be found in summer. Either or both of these influences determine 
boundaries of distribution toward the poles or higher altitudes. 
Limiting maximal temperatures for survival commonly occur in sum- 
mer and minimal chilling necessary for reproduction and growth will 
be found in winter. These influences determine boundaries of dis- 
tribution toward the equator or low altitudes. On the basis of the 
foregoing generalities Hutchins (1947) has delineated four basic types 
of zonation in geographical distribution and has discussed them par- 
ticularly in relation to the ranges of marine animals. As we shall see, 
in these common types of temperature zonation as well as in more 
irregular situations, failure to meet the thermal requirements listed 
above may occur at any stage of the life cycle and at various seasons 
of the year. 

Control by Extremes. The simplest type of control of geographical 
distribution by temperature extremes is that in which the polar limit 
of the range of a species is determined by the minimum temperature 
in winter and the equatorial limit is fixed by the maximum summer 
value. Many illustrations of this type of control will occur to the 
reader and can be found in treatments of plant and animal geography. 
Sometimes the lowest temperatures tolerated by tropical species are 
encountered far to the south (in the northern hemisphere), and the 
highest temperatures tolerated by arctic species are found well to the 
north. There is evidence, for example, that the southward ( and east- 
ward) distribution of three flowering alpine perennials in Scandinavia 
is correlated with the maximum temperatures in summer as indicated 
in Fig. 5.12. Although the exact causal relationship in such instances 
is not known, it is possible that the protoplasm of the northern species 
is adversely affected, that respiration is increased faster than assimila- 
tion, or that temperatures are too high for the proper transport of as- 
similated products (cf. Went, 1950). 

Intensive study of certain species has revealed the fact that control 
by extremes may involve unexpected complications and that other 
aspects of temperature influence may be in operation simultaneously. 
A multiphased action of temperature will probably be found in many 



Control by Extremes 




Dakl, 1951 

FIG. 5.12. Locations of the most southern and eastern records in Scandinavia 
of three flowering alpine perennials and the maximum summer temperatures, 

x Lactura alpina, correlated with 29 C isotherm. 

o Ranunculus platanifolius, correlated with 27 C isotherm. 

Saxifraga foliolosa, correlated with 23C isotherm. 



162 Temperature 

more species when they have been more closely investigated. Ac- 
cording to Iversen (1944), for example, the northern and eastern 
limit in Europe for the growth and reproduction of the ivy, Hedera 
helix arborea, is determined by the minimum winter temperature in 
situations where the average temperature for the coldest month is 
1.5C or below, but in areas with less severe winters the northern 
limit is fixed by the occurrence of sufficient summer heat as indicated 
by the average temperature for the warmest month (Fig. 5.13). 



-1-10 

11 

12 

i 14 
w i5 



ro 

X 17 



| 19 



21 
22 



o? 



Pert 



6 5 4 3 2 1 0-1-2-3-4-5-6-7-8-9 -10 
Average temperature for January, C 

Iversen, 1944 

FIG. 5.13. Temperature conditions at localities in Europe in which the occurrence 
and condition of the ivy, Hedera helix f. arborea, have been investigated. 

Ivy sets fruit regularly. 

O Ivy sets fruit occasionally, 

o Ivy occurs but does not set fruit. 

x Ivy does not occur. 

The harmful action of high temperature is not necessarily limited 
to midday hours in summer nor that of low temperature to the coldest 
night in winter. We have seen in the previous chapter that winter 
killing of vascular plants is sometimes due to the excessively warm, 
dry wind during periods when the ground is frozen. For many plants 
the danger of injurious low temperatures occurs in the spring after 



Control by Extremes 163 

germination, and at that time killing will result from temperatures 
that are not nearly as low as those which the plant successfully with- 
stood during the winter while it was in a dormant condition. An un- 
expected temperature relationship is exhibited by certain plants in 
northwest Europe that can extend their range farther poleward in the 
interior of the continent than they can along the sea coast. The ex- 
planation is that the milder winter and earlier rise of temperature 
near the ocean stimulates the plants here to earlier germination, but 
since the spring temperatures rise more slowly the seedlings may be 
killed by late frosts. 

Among those species whose geographical range is limited by tem- 
perature extremes we find a great variation as to the part of the life 
cycle that is chiefly concerned, The limitation is brought about by 
the most susceptible stage; the boundary of distribution is determined 
by the weakest link in the life cycle in respect to temperature tol- 
erance. Other life stages may extend temporarily beyond the area 
of permanent existence. 

We often think of the youngest organisms as the most sensitive to 
harmful influences in the environment, and this is indeed sometimes 
true in respect to temperature. The early stages of development 
often exhibit a very narrow range of temperature tolerance and may 
thus represent the "heel of Achilles" for the species as far as damage 
from excessive heating or chilling is concerned. For example, fish 
eggs characteristically show a greater susceptibility to harm from 
temperature than adult fish of the same species. The eggs of birds, 
although a special case because birds are homoiotherms, will tolerate 
only an extremely narrow fluctuation in temperature and frequently 
die as the result of unusually cold or hot weather. In some animals, 
on the other hand, the egg is far more resistant to temperature ex- 
tremes than any other stage. In other species the most resistant stage 
is found later in the life cycle and may be represented by a cyst or 
pupa. The seed of the higher plants similarly represents a tough, 
resting stage and one that is characteristically able to withstand a 
range of temperature far greater than that tolerated by the adult plant. 
Sprouts can often survive a frost that would critically injure the re- 
productive tissues of the mature plant. 

Very frequently the greatest susceptibility to heat or cold damage 
lies in the intermediate stages of growth. The period after the egg 
has hatched or after the seed has germinated is a critical one in some 
species; it represents the stage at which pioneering individuals at the 
margins of their range will be killed off by adverse temperatures. 
The fate of copepod populations carried by currents each year from 



164 Temperature 

the Gulf of Maine into the Bay of Fundy serve as an illustration of 
this situation. Fish and Johnson (1937) report that adult copepods 
brought into the Bay of Fundy find the temperature tolerable, breed 
successfully, and produce eggs that are viable under the existing 
thermal conditions. The temperature remains too low for the ensuing 
larval stages, however, with the result that the new generation is 
eventually killed off. The copepod population is consequently re- 
duced until the next year when it is again renewed by currents from 
the Gulf of Maine. Sometimes the harmful action of temperature is 
quite indirect. Although the free-swimming life of the oyster, Cras- 
sostrea virginica, is extended from 7 days at 25C to 21 days below 
20C, the animal develops perfectly well at the latter temperature. 
The principal significance of low temperature for the larvae is that it 
prolongs pelagic life and hence increases the period during which 
the larvae fall easy prey to enemies (Thorson, 1950). 

In some species the adult stage is more sensitive to extremes of 
temperature than any of the younger stages. Excessive heat or cold 
may reduce the vitality of the plant or animal as a whole, or it may 
cause damage to specific tissues or functions. The organs of repro- 
duction are frequently the most susceptible. For example, the north- 
ward distribution of peach trees in the United States is limited by 
damage to fruit buds by late frosts in the spring, although the other 
parts of the plant remain quite uninjured. The foregoing discussion 
will suffice to illustrate the fact that extremes of heat or cold may kill 
off animals or plants at whatever season the most vulnerable stage of 
development occurs. 

Control by Need for Minimum Amount of Heat. In order that a 
species maintain itself in a given locality, the temperature at some 
season must rise above the threshold, that is, above the minimum 
at which the vital processes of the organism can go on, and it must 
remain above that value for a period sufficient to allow growth and 
reproduction to be completed. The great variation in the length of 
the frostless season within the United States is indicated in Fig. 5.14. 
Many organisms require temperatures considerably above freezing 
at some season, but occasionally we find instances of plants or animals 
that can get along with remarkably little heat. Lichens were found 
growing on Antarctic nunatacs (rocky peaks protruding through the 
ice) where extreme cold prevails for much of the year and the tem- 
perature rises above 0C on very few days. Similarly, a rich and 
varied marine invertebrate fauna and associated flora are known to 
grow and to reproduce along the coast of Greenland where the water 
temperature never exceeds 0C. 



Control by Need for Minimum Heat 165 

Above the thermal threshold it is generally true that, the higher the 
temperature, within limits, the shorter the period needed for the same 
amount of development. This fact led to the belief that the heat re- 
quirements for the growth of an organism from its youngest stage to 
maturity, or for the completion of any other life process, might be cal- 
culated quite mechanically as a "heat sum" (or "thermal constant") 
expressed as "degree days" or "degree hours." The heat sum was 
obtained by finding the threshold temperature for the process con- 
cerned and summing the differences in degrees between this value 
and the average temperature on each day until the completion of the 
process. Thus the heat sum required for the flowering of corn plants 
in Ohio was found to lie between 660 and 1050 degree days, basing 
the calculation on the number of centigrade degrees above 6C 
recorded for the temperature each day. The lengths of the life his- 
tories of many insects have been shown to depend upon a relatively 
constant temperature sum over a considerable range of temperatures, 
and hence of velocities of development (Allee et al., 1949, Ch. 6). 



< 90 days 
U| 90 - 120 
E3 120 -150 
33 150 -180 
~~\ 180-210 



J~\ J^^^^^ 

EZ3210-240 \ jF 
~~~ V V 




Data from Atlas of American Agriculture, U. S. Department of Agriculture, 103 

FIG. 5,14. Average number of days between last killing frost in spring and first 

killing frost in fall. 

Although the results of temperature summation agree generally with 
the fact that a given type of plant or animal requires longer to mature 
in a cold than in a warm climate, closer scrutiny shows that, in many 
species at least, temperature dependencies are far more involved than 



166 Temperature 

are indicated by heat sums. In the first place the heat sum for a 
process is constant only for that range within which there is direct 
proportionality between growth rate and temperature. Lack of such 
proportionality is usually found near both the upper and the lower 
limits of tolerance. Indeed, as mentioned earlier, above certain 
"optimal" temperatures the rates of many metabolic processes ac- 
tually decrease. Furthermore, since heat sums are based on average 
daily temperatures, they do not take into account the possible special 
effects of the maximum or minimum daily values, night temperatures, 
or differences between diurnal and nocturnal temperatures. 

The critical importance of these more detailed aspects of tempera- 
ture in controlling life processes is discussed in greater detail by 
Daubenmire (1947) particularly in relation to land plants. Some 
plants exhibit normal responses only when grown in temperatures 
that fluctuate with a regular diurnal rhythm a phenomenon known as 
thermgjyyiodism. And to make matters more complicated, the pre- 
cise action of temperature may be further modified by the intensity 
of light and the length of day. Went ( 1950) points out that no heat 
sum can account for the ripening of the tomato. In addition to 
proper light intensity, length of day, and day temperature, the tomato 
plant sets seed only if exposed to a series of five or more nights dur- 
ing which the temperature remains above 15C. These findings are 
compatible with the general principle of minimum heat requirements, 
but they show that in certain land plants at least the matter is far 
more complicated than first supposed. 

Control of the geographical distribution of many animals can sim- 
ilarly be related to the need for a minimum amount of heat. The 
American oyster as an adult can survive at temperatures from 32C 
down to the freezing point of salt water. A temperature above 15C, 
or higher in some races, is necessary for spawning to take place in this 
species, and good development of the larvae requires water above 
18 or 20C. The adult lobster can live at temperatures ranging from 
about 17C down to 0C, but breeding will take place only in water 
warmer than 11C. The lower maximum temperature tolerated 
means that the lobster cannot range to the south as far as the oyster 
and the lower heat requirement for lobster reproduction produces a 
marked difference in the breeding range of the two animals. For a 
complete study of the effects of temperature on distribution the ther- 
mal conditions in each bay and inlet should be investigated, but the 
range of temperature along the coast is sufficient to illustrate the gen- 
eral influence of this factor. 

A glance at Fig. 5.15 will show that north of Cape Cod coastal 



Control by Need for Minimum Heat 167 

temperatures are generally lower than 18C until the Gulf of St. Law- 
rence is reached where surprisingly warm conditions occur in summer 
in the shallow areas around Prince Edward Island. Temperatures in 
the central part of the Bay of Fundy are prevented from rising above 
11C by the intense tidal stirring, but the stratified water along the 
outer coast of Nova Scotia becomes considerably warmer than this. 
Correlated with these temperature conditions we find that in New 
England oysters grow naturally in commercial quantities only as far 
north as Narragansett Bay but "seed" oysters may be transported to 
Cotuit and elsewhere on Cape Cod, where they are fattened for mar- 




Modified from Ackerman, 1941 

FIG. 5.15. Regions of abundant occurrence of lobsters (small dots) and oysters 
(large dots) in relation to August surface temperatures as indicated by the loca- 
tion of the commercial catch in 1934. 

ket. Oysters again become sufficiently numerous to market in the 
littoral zone around Prince Edward Island and in certain warm bays 
of Cape Breton and northeast New Brunswick. Lobsters, on the 
other hand, breed successfully in abundance all along the coast of 
Maine, on the outer coast of Nova Scotia, and into the Gulf of St. 
Lawrence, but these shellfish are not taken in commercial quantities 
within the Bay of Fundy. Although lobsters wander into the Bay 
of Fundy and some of them grow to a large size, the lack of water 



168 Temperature 

of sufficient warmth prevents the establishment of a native breeding 
population despite the fact that the animal's range extends much 
farther north in situations where the heat requirements are met. 

The northward distribution of turtles in inland waters is similarly 
determined not by the extremes of temperature but by the need for 
the minimum amount of heat for the completion of the life cycle. 
Since turtles burrow in the mud during the winter, the extreme mini- 
mum air temperature at that season is of no consequence. But these 
reptiles require the heating action of the summer sun to incubate their 
eggs. The snapping turtle, for example, lays its eggs in the sancl dur- 
ing the month of May, and sufficient heat must reach them during the 
summer in order for the young turtles to hatch out successfully in 
September. The northerly distribution of turtles is correspondingly 
limited by this long heat requirement to the upper or central part of 
the United States, but snakes can extend their range well over the 
Canadian border because heat for the incubation of their eggs is 
needed for only a short period. 

Differential control of geographical range is also illustrated by the 
distribution of frogs, whose temperature requirements are widely di- 
vergent. Some species of frogs seem to need little more heat than 
that necessary to thaw out their ponds just long enough for reproduc- 
tion. The eggs of the wood frog, Rana sylvatica, develop at a tem- 
perature as low as 2.5C, and the larval stage requires only 60 days. 
This species extends northward in Canada to the mouth of the Mc- 
Kenzie River. The pickerel frog, Rana palustris, on the other hand, 
must have a temperature of 7.5C and a period of at least 90 clays for 
its development. The range of this species is correspondingly limited 
to the latitude of James Bay. In contrast, a third species, Rana clam- 
itans, will not develop until the temperature exceeds 11C, and its 
range extends only slightly above the southern boundary of Canada. 

Control by Need for Chilling. Some organisms require a certain 
period of cold weather for their life cycles to be completed. No one 
would plant tulips or crocuses in June and expect them to bloom in 
August. These plants come into flower only after the bulbs have 
passed through a winter period of low temperature. The dormant 
buds of certain fruit trees and berry plants similarly require a period 
of chilling before they will flower successfully. Experiments have 
shown, for example, that some types of blueberries require an ex- 
posure of 800 hours to temperature below 7C before the dormant 
buds will develop. In other species the initial formation of flower 
buds will take place only at low temperatures. 



Control by Need for Chilling 169 

The seeds of some plants must be chilled under moist conditions 
before they will germinate properly, In other species, such as winter 
wheat, satisfactory development takes place only if a period of chilling 
occurs during or after germination. It has been discovered that the 
seeds of certain plants must undergo two successive cold intervals 
before the seedling will grow (Barton, 1944). This requirement 
means that under natural conditions development of the plant can go 
forward only after the second winter. 

In nature all species requiring low temperatures are limited in their 
altitudinal distribution down the sides of mountains and in their 
latitudinal extension toward the equator. This cold requirement also 
imposes serious restriction on the successful cultivation of temperate 
varieties of fruit trees in warm climates. Although little is known of 
the physiological mechanisms upon which the need for chilling de- 
pends, the empirical knowledge is put to practical use through 
vernalization, that is, the artificial exposure of seeds to cold. Such 
procedure allows certain crop plants to be grown farther north and 
also farther south than would otherwise be possible. 

Another need for low temperatures results from the varying effect 
of this factor on anabolic and catabolic processes. We have seen that 
sufficient heat must be available to plants and animals so that the 
growth and reproductive processes may make up for the destruction 
of tissue materials and the death of individuals. As temperatures rise, 
most vital reactions are accelerated, but sometimes at very different 
rates. If catabolic processes are speeded up disproportionately, the 
organism will suffer rather than gain from high temperatures. Above 
certain temperatures the respiration rate of most plants becomes 
higher than the rate of photosynthesis. Food manufacture is conse- 
quently curtailed, and growth, reproduction, and the accumulation 
of food reserves may be inhibited. The range of plants toward low 
latitudes and low altitudes and their seasonal activity are limited to 
areas and periods when temperatures remain low enough to permit 
a favorable photosynthesis-respiration relationship. 

Although the need for low temperatures during part of the life cycle 
has been extensively studied in relatively few animals, there is little 
question that failure to meet this thermal requirement limits the dis- 
tribution of some species. Desert mammals and insects that are 
forced to aestivate by high temperatures would not be able to con- 
tinue active life if some portion of the year did not have a cooler cli- 
mate. The gemmules of sponges hatch at a much lower temperature 
and in a much shorter time after a period of freezing. The eggs of 



170 Temperature 

the white mayfly, Ephoron album, must be chilled to within a few de- 
grees of freezing for several weeks (or subjected to a few days of 
actual freezing) for good hatching to be assured. As a result, this 
insect cannot extend its range in the eastern part of the United States 
farther south than 40 N latitude. 

The eggs of the Australian grasshopper will not continue growth 
beyond a certain stage unless they are exposed to low temperatures 
within the range that will permit diapause development to proceed 
rapidly (5-13C), and later, to high temperatures (17-35C) which 
permit morphogenesis to go forward (Fig. 5.16). This dependency 




20 



80h / V^ A D / 1 16 S 

-148 

12 J 

0) 

^0 | 
40h / \ \ / -H8 I 

6 f 
20h" >T\ -J4 1 



5 10 15 20 25 30 35 

Temperature, *C 

From Andreu'attha, 1952, copyright Cambridge Univ. Press 

FIG. 5.16. The influence of temperature on (A) diapause development and (B) 
post-diapause morphogenesis in the embryo of the grasshopper, Austroicetes 
cnwiata. Solid line: eggs from South Australia; broken line: eggs from West 

Australia. 



on chilling is nicely attuned to the normal life cycle of this grasshop- 
per. Since the insect reaches the diapause stage at the beginning of 
winter, it is in the condition to endure best the severest cold weather 
of the year and also to take advantage of low temperatures for the 
completion of diapause development. Other species in which dia- 
pause occurs during later stages of the life history exhibit a different 
timing in their adjustments to the ecological conditions of their habi- 
tat. The eggs of the moth, Euproctis, hatch during July, and dia- 
pause does not take place until the larvae have reached the second 
or third instar. By the time the animals enter diapause autumn has 
arrived and the species overwinters in the larval stage. 

In New England the gypsy moth lays eggs in summer, but develop- 



Control by Need for Chilling 171 

ment will normally start again only after chilling, or not until the fol- 
lowing spring. This timing not only fits the life cycle of the moth 
but also is most fortunate for the survival of the vegetation in areas 
where this pest is abundant. If reproduction continued throughout 
the late summer and autumn months, the trees upon which the cater- 
pillars depend for food would have no opportunity to recover from 
the spring and early summer inroads of feeding. Even in species 
without a recognized period of dormancy distribution toward warmer 
regions may be limited by the fact that warm conditions inhibit cer- 
tain necessary processes long before temperatures are reached that 
would be immediately lethal. The southern ( equatorward ) bound- 
ary for the barnacle, "Bdlamis balanoides, appears to be determined 
by the isotherm for a minimum surface temperature of 7.2C (Hutch- 
ins, 1947). Evidently this barnacle requires a certain period of low 
temperature in order to complete its reproduction successfully; this 
conclusion is supported by the fact that the species breeds during the 
summer in the northern part of its range, but breeds only during the 
winter in the southern areas of its distribution. 

The foregoing consideration of the limitation of distribution by 
thermal requirements has called attention to the complexity of the re- 
lationships involved. Temperatures that are harmful, or that are 
necessary, differ widely with species, age, and physiological condi- 
tion. Resistant stages of a species may be found far beyond the 
boundaries of the area within which all parts of the life cycle may 
be carried on. For marine bottom invertebrates, for example, Thor- 
son (1950) has delineated three areas of distribution: (1) an exten- 
sive area throughout which the adult animal may live at least vegeta- 
tively; (2) a smaller area within the first in which the animal will 
ripen its sexual products and spawn; and (3) a still smaller area 
within the second in which the embryos and larvae will develop 
successfully. Other types of animals and plants have different pat- 
terns of temperature sensitivity at different life stages. As we have 
seen, the effect of temperature may vary for different life processes 
in the same organism or even for the same process in different parts 
of the organism. Each species of animal or plant can live only in 
those regions in which the temperature pattern is tolerable and ade- 
quate for the needs of the species. The organism is restricted to 
habitats in which the annual, seasonal, and diurnal fluctuations in 
temperature occur at such times and in such magnitudes as to allow 
all life processes to be completed but do not occur so as to cause 
serious harm. The intriguing interdependencies outlined here cer- 
tainly need and invite further investigation by ecologists. 



172 Temperature 

Results of Temperature Limitation 

One far-reaching consequence of the action of temperature in the 
various ways that have been reviewed is to impose a north-south 
gradation on the distribution of many animals and plants both in 
water and on land. The ranges of species that are controlled pri- 
marily by temperature, rather than by moisture or some other ecologi- 
cal factor, have boundaries tending to run generally east and west. 
Latitudinal zonation is considerably modified, however, by tempera- 
ture anomalies due to currents in the ocean, and to differences in alti- 
tude, proximity to oceanic areas, and other influences on land. Across 
the United States, for example, less severe cold is experienced along 
the Atlantic and Pacific coasts than in the center of the continent, and 
cold weather occurs farther south in the mountain areas. The ranges 
of many species correspondingly extend farther north along the coasts 
but are restricted to more southerly regions in the Rockies and in 
the Appalachians. On the other hand, in northwestern Europe 
species for which the northern boundary is determined by summer 
warmth range farther north toward the interior of the continent be- 
cause the temperature in summer is higher there than on the coast. 

There are no temperature boundaries to which the ranges of all 
plants and animals conform. The complex influence of thermal con- 
ditions must be investigated for each species separately before gen- 
eralities applying to groups of animals and plants can be established. 
As we go north or south species drop out irregularly and their ranges 
overlap. The pines of the north, for example, extend into the realm 
of the palms of the south. The live oaks, typical of southern United 
States, overlap in their distribution the white oaks, whose center of 
distribution lies farther north. The quail extends its range northward 
into New Hampshire, but the grouse, a similar type of bird common 
in Canada, spreads southward as far as Virginia. 

The same sort of overlapping occurs with aquatic organisms. Of 
40 species of marine invertebrates common along the eastern coast of 
the United States, 16 have their northern boundary at Cape Cod, 14 
have their southern boundary at the same point, but 10 species occur 
on both sides of the Cape. In the study of geographical distribution 
as influenced by temperature, we see again the necessity for the ana- 
lytical approachfor dealing primarily with the mechanism of control 
of individual species. 

Special Cases of Common Boundaries. Only in special instances 
do a large number of plant and animal species share a common ther- 



Special Cases of Common Boundaries 173 

mal boundary for their distribution. Under unusual circumstances 
the horizontal temperature gradient may change abruptly. At the 
northern edge of the Gulf Stream, for example, the temperature may 
drop as much as 5C within a few miles-an extreme thermal change 
for the oceanic environment. We are not surprised that many species 
inhabiting Sargasso water have northern boundaries and species in- 
habiting continental slope water have southern boundaries at the Gulf 
Stream. Similar sudden changes of climatic conditions occur on 
land in relation to unusual topographical changes, such as an abrupt 
change in altitude at the edge of a plateau. In these situations a 
considerable number of plants and animals share common boundaries 
based primarily upon the temperature factor. 

Frost Line. The frost line on land constitutes another special 
thermal situation but one in which there is no sudden change in tem- 
perature geographically, The fact that the tissues of animals an<} 
plants in active condition freeze not far below 0C results in the 
occurrence of a boundry of general importance at the frost line. 
All plants and all animals that are not frost hardy and that do not 
have modes of escape are limited in their poleward and altitudinal 
distribution by the harmful results of free/ing. At the same time 
any plants or animals dependent upon these frost-sensitive species 
will drop out. The palm trees are an example of plants that are 
unable to stand a continued frost. In North America the natural 
range of the true palms does not extend north of the central part 
of Florida, the Gulf coast, and the southern part of California. 
Smaller plants and animals associated with the palms are necessarily 
limited in their distribution to essentially the same boundaries. 

Tree Line. Other abrupt changes in the fauna and flora commonly 
occur where one major type of vegetation gives place to another, since 
the dominant vegetation influences the presence of many subordinate 
species of plants and animals. An outstanding instance of common 
boundaries due to the dependence of many species on the type of 
vegetation is seen at tree line. Here again there is no sudden 
change in the distribution of temperature. When thermal and other 
conditions have been modified to such a point that trees no longer can 
grow, an important change in the whole ecological situation takes 
place. In any forested region trees have a profound controlling in- 
fluence on the subordinate plants and on the associated animal life. 
Thus the environmental situation that limits the trees indirectly sets 
boundaries to the distribution of many other species as well. Tree 
lines dependent in part at least on temperature occur across the con- 
tinents at the poleward margins of the forests and also on mountain 



174 Temperature 

slopes at the upper limits of forest vegetation. We may consider first 
the northward boundary of timber in Canada as an example of a 
continental tree line. 

A traveler going north in the eastern part of North America passes 
from the southern region of palm trees into the belt of southern 
pines and then into the "central hardwoods," dominated by white oaks 
and hickories. The central hardwoods extend into southern New 
York, Connecticut, Rhode Island, and maritime Massachusetts. In 
east central Massachusetts and up the valleys in New York, Vermont, 
and New Hampshire the vegetation changes to the "transition forest," 
in which the maple, red oak, ash, and black birch are the most 
abundant species. In northern New York, central Vermont, and 
New Hampshire the forest becomes the "northern hardwood" type, 
with maple, beech, and white and yellow birch predominating. 
When the traveler reaches northern Maine and the corresponding 
parts of Ontario, he finds himself in the spruce forest. Here red 
spruce, firs, and white and yellow birch are the common trees. Still 
farther north, about at the southern edge of Hudson Bay, the trees 
have become so small that the Indians referred to the area as the "land 
of little sticks." Then the forest stops. 

Although temperature is believed to play a major role in determin- 
ing the poleward boundary of forest vegetation, it is difficult to de- 
termine just how this factor operates. We know that tree line is not 
related primarily to the coldness of winter since even in Siberia, 
where extremely low temperatures are recorded, heavily forested 
regions are found. Nor is the tree growth determined directly by 
the occurrence of certain maximum temperatures in summer, since in 
Labrador and in Alaska, far beyond the tree line, temperatures of 
32 to 38C have been recorded. High daily maxima are beneficial 
since they hasten the melting of snow. In order to survive trees must 
have sufficient heat for growth in the interval between the arrival 
of effective temperatures in the spring and their disappearance in the 
fall. In Alaska, and also on the San Francisco mountains, the conifers 
reach but do not pass the July isotherm of 10C (Griggs, 1946). On 
the other hand, the position of the tree line in Scandinavia appears 
to be chiefly influenced by the occurrence of a season with sufficient 
heat for seed germination, especially of the birch. 

In some regions the curtailment of tree growth is brought about by 
excessive soltfluctionthe heaving of the soil by frost action. Trees 
are tilted and their roots broken by severe solifluction, and in such 
situations this indirect action of climate may be more significant 
than its direct effect. The occurrence of permafrost (a stratum of 



Life Zones 175 

permanently frozen soil) prevents the penetration of tree roots, and, 
although trees will grow in an unfrozen stratum over permafrost, its 
presence usually increases the severity of solifluction. Raup (1951) 
regards timber line in Alaska as occurring at the point where frost 
action prevents the white spruce from surviving in numbers on the 
uplands. Isolated stands of spruce trees are found growing hundreds 
of miles north of the recognized tree line in areas where local drain- 
age conditions result in little or no solifluction. 

Fortunately, the student of ecology may observe the profound 
effects of the tree line without the necessity of visiting Alaska or the 
vicinity of Hudson Bay. In the United States many of the accessible 
mountains both in the east and in the west rise to sufficient heights 
to display altitudinal tree lines. At the upper edges of the coniferous 
forest, the spruce, fir, and birch become progressively reduced in size 
until they culminate at tree line in a dense scrub growth or dwarfed 
individual specimens. The actual altitude at which tree line occurs 
differs widely according to local circumstances. The harmful effects 
of strong wind, including especially excessive evaporation, are be- 
lieved to restrict tree line to lower altitudes on exposed mountain 
peaks than would be the case if temperature were acting alone 
(Griggs, 1946). Physical influence of ice formations, avalanches, 
landslides, and solifluction may exert a further control locally (Fig. 
5.17). Thus on different slopes of Mt. Washington, New Hampshire, 
described as having "the worst weather in the world," trees give way to 
alpine tundra at altitudes ranging from about 1200 m to 1740 in, 
whereas in the Rocky Mountains forests may extend to 3300 m (Fig. 
5.18). Tree line occurs at sea level in parts of Belle Isle Strait but 
at an altitude of about 300 m at Bay of Islands, Newfoundland. 

Life Zones. The progressive change in temperature and other 
climatic factors from the equator toward the poles and from the low- 
lands to the mountain peaks controls the distribution of certain major 
vegetation types and these in turn are accompanied by characteristic 
sets of subordinate plants and associated animal species. As a result, 
we may recognize a series of life zones extending across the land from 
the tropics to the polar regions, and, on a much smaller scale, extend- 
ing up mountain slopes from the warm lowlands to the alpine condi- 
tions of high elevations. Although many irregularities exist and 
although the special circumstances influencing the distribution of 
each individual species must be kept in mind, the general sequence 
of the latitudinal and altitudinal life zones is clear, and the two series 
have recognizable counterparts (Fig. 5.19). The continental life 
zones are formed by the roughly latitudinal arrangement of certain of 



176 



Temperature 



: 












FlG. 5.17. Winter view ot tret- line on Mt. Mansfield, Vermont, with spruce trees 

in foreground encased in iee. Severe frost action and other adverse climatic 

influences reduce the trees to stunted dwarfs. 




Photo by D. R. Clarke 

FIG. 5.18. General view in midsummer of tree line on Mt. Rainier, Washington, 
and of the alpine meadows above it. Tree line here is believed to be relatively 
stationary because of the occurrence of both old and young trees among the out- 
posts of the forest. 



178 Temperature 

the principal biotic formations, or bionics, that will be discussed more 
fully in Chapter 12. 

A synoptic view of the life zones of the world could be obtained 
by chartering a plane at the equator and flying toward the north 
pole, or toward the south pole. On such a trip, you would observe 
that the life zone characteristic of the equatorial region is the tropical 
rain forest. In this* zone a wide variety of trees provides continuous 
shade and humid conditions for the many species of plants and ani- 
mals that live in and under the forest. North of the tropical rain 
forest the flying ecologist comes to a life zone in which dry seasons 
alternate with rainy seasons. Farther to the north the plane passes 
over a belt of varying width of desert or grassland and thence over 
deciduous forests of oak, hickory, beech, and maple. The colder 
region to the north is heavily forested with spruce and other conifers 
that form a circumpolar life zone. At the limit of the spruce forest 
the plane crosses the tree line and reaches the tundra where dwarf 
birches or willows, scattered grasses or other flowering plants, mosses, 
and lichens struggle to keep alive on ground that is frozen for much 
of the year. Eventually all land vegetation is lost to view in a land- 
scape of universal snow and ice. If the plane trip had been made 
from the equator southward, the same general sequence of life /ones 
would have been traversed except for the gap caused by the great 
expanse of the Antarctic Ocean. 



4,000m 

3,500 
JIMBER LINE 

Spruce-Fir > 

3,000 

, ....2,500 

Submontane shruoV 
7,000 L -^ 

5,500 

4,500 

North UINTA MOUNTAIN S TAVAPUTS PLATEAU 

FIG. 5.20. Diagrammatic representation of the altitudinal life zones in the Uinta 
Basin. (Graham, 1937.) 

Without an airplane you may observe the same general series of 
life zones on a much smaller scale by climbing a mountain. If the 
mountain is a high one and located in the tropics, as is Mt Popo- 
catepetl in Mexico with an altitude of 5448 m, the basal slopes may 





Life Zones 179 

be covered by rain forest and the summit by snow. As you climb 
upward you emerge from the dense vegetation of the tropical forest 
and pass progressively through life zones of savannah, deciduous 
forest, and coniferous forest. Higher up you reach a belt of dwarf 
pine, fir and alder. Above this a life zone of grasses, lichens, and 
mosses exists, corresponding to the tundra and giving way finally to 

TABLE 13 

OUTLINE OF PLANT COMMUNITIES i\ THE UINTA BASIN AND THEIR RELATION 
TO ALTITUDIXAL LIFE /ONES (Graham, 1937) 



Upper 
altitude 



Mid 

altitude 



Low 
altitude 



Alpine zone 



Spruce-fir zone 



Lodgepole pine zone 



Aspen zone 



Juniper-pinyon zone 





S ic vcrs ia- ( 1 arex association 




Picea-Abies association 
Upper-altitude meadow 
Upper-altitude lake 


one 


Pinus Murrayana association 




Populus aurea association 
Mid-altitude valley 


ib zone 


Mid-altitude Artemisia associations 


.one 


Juniperus-Pinus association 


ub zone 


Eurotia association 
Low -altitude Artemisia association 
A triplex- Tetradymia association 
Chrysothamnus association 
Kochia-IIilaria association 
Mat Atriplex association 
Sarcobatus association 




Desert gulch 
Badlands 
Distichlis meadow 
ScirpuS'Typha swamp 
Cottonwood river flood-plain 



bare rocks, snow, and ice. Altitudinal zonation on the slopes of 
mountains in the temperate region is often equally striking but may 
not be as extensive. The report by Graham (1937) of the Uinta 
Basin of Utah and Colorado contains a detailed study of the life zones 
at different altitudes with many excellent photographs of representa- 
tive plant communities (Fig. 5.20 and Table 13). 



180 Temperature 

Although temperature relations are sometimes chiefly responsible 
for the demarcation of certain life zones, it is obvious that moisture 
is extremely influential in many instances, and other ecological factors 
such as soil condition and length of day play contributory roles. The 
positions of life zones cannot be universally correlated with any one 
influence of the environment but are the result of a complex of in- 
teracting factors. 

Nevertheless, an attempt was made by Merriam in 1894 to set up 
zones based exclusively on temperature into which the entire fauna 
and flora of North America could be divided. These were called 
biothermal zones and, unfortunately, were accepted uncritically by 
many ecologists. There are certain specific objections to the wide 
application of Merriam's scheme besides the general warning that the 
presence of an organism in a certain locality cannot be accounted for 
by saying that it is a "member" of a certain biothermal zone ( Dauben- 
mire, 1938). In the first place the heat requirement calculations for 
Merriam's zones were based on 6G as a threshold temperature for the 
germination of wheat, and therefore obviously should not be directly 
applied to other species of plants and particularly not to animals. 
The assignment of birds and mammals to zones based on environ- 
mental heat conditions is especially inappropriate since these warm- 
blooded animals carry their own temperature around with them. 

Other objections to the use of Merriam's zones appear from a critical 
understanding of the varied effects of temperature. For example, the 
number of the degree days required for development is riot constant 
over a wide temperature range. Fifty days at 2C above the thresh- 
old have a very different influence from that of 5 days at 20C above 
the threshold, although the "thermal constant" would be the same in 
both cases. Furthermore, as explained above, temperatures at seasons 
of the year other than those used by Merriam may be critical in con- 
trolling the distribution of many species. The fact that distribution 
of a species may agree with certain isotherms of the region does not 
necessarily prove a direct causal relation. The existence of many 
minor species of plants is controlled by the presence of dominant or 
accessory species. The geographical range of the dominant species 
may in fact be controlled by temperature, or it may be dependent 
primarily on the moisture factor, which in turn is influenced by tem- 
perature. 

Animal populations also may exist in characteristic zones for reasons 
that are only indirectly related, or unrelated, to temperature. Horned 
larks, for example, are found all the way from Colombia to the Arctic 
and occur in very divergent climates and hence in various biothermal 



Temperature and Moisture Together 181 

zones. The primary relationship is with the vegetation: these larks 
are found wherever broad expanses of shortgrass occur regardless of 
thermal conditions. Cone-feeding birds are similarly found wherever 
suitable conifers are growing. The range of this type of bird may 
include areas in one life zone where a northern conifer is growing and 
also in another life zone where a southern conifer is growing. It is 
obviously absurd to think of the range of this bird as primarily de- 
termined by biothermal zonation. 

Temperature and Moisture Acting Together 

A discussion of temperature as an ecological factor should not be 
concluded without calling attention again to the close interrelationship 
between temperature and moisture in the terrestrial environment. 
Emphasis has been placed on the fact that temperature affects relative 
humidity and the rate of evaporation. The converse is equally true 
that when evaporation and condensation occur, they tend to modify 
the temperature. We are accordingly not surprised to find that tem- 
perature and moisture often interact in such a way as to make it dif- 
ficult or impossible to disentangle the individual effects of these two 
factors. Does the salamander go into a sheltered place to keep cool, 
to get warm, or to avoid excessive evaporation? Added to the com- 
plication is the fact that for many organisms the indirect effects of 
these factors may be more significant than the direct effects because 
of their control of the vegetation. 

We have evidence that the influence of humidity on insects is often 
greatly modified by the existing temperature. The dual effect of 
these factors on the rate of development of the cotton boll weevil pro- 
vides an admirable illustration (Fig. 5.21). This insect pest cannot 
develop if the relative humidity is less than 40 per cent or more than 
88 per cent, no matter how favorable the temperature may be. On 
the other hand, the animal remains dormant regardless of humidity 
if the temperature is lower than 17C or higher than 39C. Within 
these ranges the speed of development depends upon the values of 
both factors. At a temperature of 28C, for example, the boll weevil 
requires 21 days to develop under a relative humidity of 40 per cent, 
but it develops in only 11 days if the humidity is between 60 and 65 
per cent. 

In the foregoing example of the combined influence of temperature 
and humidity on rate of development it is of interest to consider what 
the limiting factor is. Picking the point on the diagram representing 
a temperature of 24C and a relative humidity of 55 per cent, we see 



182 Temperature 

that the speed of development will be increased if either the tempera- 
ture or the humidity is raised. In this instance more than one limit- 
ing influence is present. In certain situations, two (or more) in- 
dependent factors may be unfavorable and both of these must be im- 
proved before the organism can continue its growth or extend its 
range. In other situations, the limiting effect of one factor depends 
upon the value of another, as in the development of the boll weevil. 
The two factors acting together produce the limitation, and may be 
referred to as a "limiting combination." The ecologist should always 
be on the lookout for possible multiple adverse or modifying in- 
fluences in the environment. Limiting combinations are probably in 
operation under natural conditions more frequently than is now real- 
ized ( Shelf ord, 1951). 



-Absolutely 

fatal 

^Dormancy 
or death 
-Dormancy 

-No 
development 

-21 days 
18 days 
16 days 
12 days 
11 days 




-10- 



Absolutely 
fatal 



10 20 30 40 50 60 70 80 90 100 

Mean relative humidity, % 

FIG. 5.21. Generalized scheme indicating the interaction of temperature and 
humidity in controlling dormancy and the number of days required for develop- 
ment in the cotton boll weevil. (By permission from Animal Ecology by Chap- 
man, 1931, McGraw-Hill Book Co.) 

An illustration of the combined action of temperature and moisture 
on vegetation is the effect of these factors on the form of growth of 
plants and on the life form of the whole plant. The climate exerts a 
major control over the type of plant that can exist in each region, and 
consequently the life form of the vegetation is to a certain extent an 



Temperature and Moisture Together 



183 



expression or an indicator of climate. This idea has been extensively 
developed by the Danish botanist Raunkiaer (1934), whose work 
should be consulted for a thorough treatment of the subject. Raun- 
kiaer stresses the significance of the adaptations of buds and shoot 
tips for withstanding adverse temperatures and surviving drought. 
On this basis Raunkiaer delineates different groups of plants. Trees 
and shrubs are placed in one group in which the surviving buds pro- 
ject into the air. The perennial grasses are included in another group 
in which the buds are situated on or near the soil surface. Plants 
with bulblike buds that are protected during adverse seasons by being 
buried in the ground are placed in a third group. 



Dry cold 



Wet cold Dry cold 



Wet cold 





Snow and ice 


s 


Tundra 


0) 


Taiga 


0) 




D 












a 
1 


^52 
E8 

53 
*S 


i! 


rassland 


1 
,? 


/> 
o> 



C 




wi C 
<l> rtJ 


CO 











Temperature efficiency 


Snow and ice 


Muskeg 


Podsols 




Desert soils 


CO 


</> 


CO 

'5 

c 
'3 



Gray-browns 


8 

| 
m 


8 


Red-and- 
yellow soils 


Red soils 
Laterites 



Dry hot Wet hot Dry hot Wet hot 

Precipitation-evaporation ratio Precipitation-evaporation ratio 

VEGETATION SOIL 

FIG. 5.22. Schematic representation of the interaction of temperature and 
moisture effectiveness in controlling the general distribution of vegetational life 
forms and soil groups in North America. (Modified from Thornthwaite, 1931.) 

Deciduous trees, conifers, shrubs, succulent xerophytes, mosses, 
grasses, annual plants, and epiphytes represent contrasting life forms. 
As we have seen the major life zones are distinguished by the life form 
of the vegetation. Plants of distinctive life form characterize the 
major plant communities, and these communities are the dominant 
feature of the biotic formations, or biomes, of the world, as described 
in Chapter 12. 

Thornthwaite ( 1931 ) has worked out indices of the effectiveness of 
thermal and moisture elements of climate as related to the range of 
temperature and the rate of evaporation in influencing the distribution 



184 Temperature 

over the continent of certain vegetational life forms. The scheme 
presented in Fig. 5.22 shows in diagrammatic fashion that above a 
certain value of thermal efficiency, moisture tends to be more re- 
sponsible than temperature for the control of life form. In colder 
climates temperature is generally more important. Soil groups are 
influenced in a similar way by the climate as well as by the vegeta- 
tion. With low precipitation-evaporation ratios and high tempera- 
tures the moisture factor is more significant than the heat factor. In 
the far north the temperature is by far the more critical, but in inter- 
mediate regions moisture and temperature play more nearly equal 
roles in determining soil groups. Here then we have another illustra- 
tion of the extremely wide direct and indirect influences of the tem- 
perature factor in the ecological adjustments of both plants and ani- 
mals. 



6 

Light 



The chief natural sources of light are sunlight, moonlight, starlight, 
and the light from luminescent organisms. In this chapter we shall 
deal primarily with visible light, but since the sun's emission also in- 
cludes ultraviolet and infrared radiation these components of sunlight 
will also be considered. Practically all the energy of importance for 
organisms under natural conditions is derived directly or indirectly 
from the sun. It is true that man has learned to obtain energy from 
other sources such as tides, and more recently from nuclear fission, 
but man like other organisms depends primarily on the sun for his 
main supply of energy. 

The radiation from the sun produces a direct heating effect, and 
also produces photochemical transformations. After these transfor- 
mations have been completed, the energy appears in the form of heat. 
Accordingly, all the sun's energy eventually ends up as heat, but cer- 
tain portions of the radiation from the sun take part in vital photo- 
chemical processes before becoming heat. 

Light as an ecological factor is generally highly directional. It 
differs in this respect from the temperature factor since heat often 
reaches the living organism from many different directions at the 
same time. Light is extremely variable. It changes over a tre- 
mendous range, often very rapidly. Many organisms can respond to a 
value of light that is only one ten-billionth of full sunlight. Although 
we often think of temperature as varying widely on the earth's sur- 
face, the magnitude of its variation is extremely small compared to 
that of light. Light can change from a value near zero to its maxi- 
mum within a few tours. 

Light is essential for most plants and animals, though some can do 
without it. For the continued existence of organisms two require- 
ments must be met. First, light must not be so strong as to cause 
serious harm at any stage in the life history. We shall see that a con- 

185 



186 Light 

siderable variation exists in the upper limits of tolerance to the light 
factor. Second, for those animals and plants that require light it must 
be sufficient in intensity and in duration. The intensity of the light 
must be above the threshold for the organism concerned, and the total 
amount of light received during the period when it is needed must 
be adequate. 

DISTRIBUTION OF LIGHT 

In discussing the distribution of light it is convenient to start with 
sunlight as the chief source and trace the radiation as it passes through 
the air and water environments. The magnitude of the solar radia- 
tion as it reaches the outer atmosphere referred to as the solar con- 
stant has the value of about 1.9 g-cal/cm 2 /min. At sea level the in- 
tensity of solar radiation averages about 1.5 g-cal/cm 2 /inin, If the 
radiation received from the sun were evenly distributed over the sur- 
face of the globe, it would be sufficient to melt a layer of ice 35 m 
thick during the course of a year. At a latitude of 44 N the energy 
received on the earth's surface from the sun is equivalent to the light 
that would be produced by hanging a 250-watt lamp over each square 
meter of the ground. 

We know very well, however, that the radiation from the sun is not 
evenly distributed cither in time or in space. We wish to inquire 
what its variations both in quality and in quantity are. Light changes 
in spectral distribution and in angular distribution. The light factor 
also varies in intensity and in duration, resulting in differences in the 
total amount of light falling on each unit of surface for each unit of 
time, such as a day or a month. The changes in the light factor in 
these respects will be examined as they affect the terrestrial and the 
aquatic environments. 

Light on Land 

Spectral Composition. The spectral distribution of light as it 
reaches the earth's surface is shown in Fig. 5,3. Authorities differ as 
to the exact wavelength limits to be assigned to the different portions 
of the spectrum. Roughly speaking, radiation of wavelength longer 
than 7600 Angstrom units is considered to be infrared, and that of 
wavelength shorter than about 3600 A is designated as pltra violet. 
Almost one-half of the total emission of the sun is infrared radiation, 
and almost one-half is visible light. These proportions remain ap- 
proximately the same at the earth's surface, regardless of the total 
intensity of sunlight. The ultraviolet component, however, is always 



Intensity of Light 187 

only a small percentage of the total radiation, and it may be reduced 
to immeasurably small quantities under certain circumstances. 

Intensity of Light. The intensity of light reaching the earth's 
surface varies with the angle of incidence and with the amount of ab- 
sorption by the atmosphere and by obscuring features. The lower the 
altitude of the sun, the smaller is the angle of incidence and the longer 
is the path of the light through the atmosphere with corresponding 
reduction in intensity. Changes in the sun's altitude result from dif- 
ferences in latitude as well as from changes in the season and in the 
time of day. The greatest intensity of sunlight occurs at positions 
on the earth's surface and at times at which the sun is most nearly 
overhead. At higher latitudes, the intensity of light is correspond- 
ingly reduced. At 50 N latitude, for example, during the period of 
the equinox in March and in September when the day is everywhere 
12 hours long, the intensity of sunlight is only about one-half of what 
it is at the equator ( Fig. 6.1 ). 




FIG. 6,1. Total solar radiation ( g-eal/day/cm 2 of horizontal surface) on March 
21 with average cloudiness. (Modified from Kiiuball, 1928.) 

Latitude thus has a definite effect; but other factors may have much 
greater influence upon the light factor. Moisture, clouds, and dust 
in the atmosphere have a profound and irregular effect in reducing 
illumination. Living organisms may also act to diminish the in- 
tensity of daylight, as is clearly shown by the forest vegetation, and 
this represents a reciprocal action in which the inhabitants modify 
the light factor in their own environment. 

Different forest communities vary widely in the degree to which 
they diminish the sun's radiation. Cottonwood (poplar) trees tend 



188 Light 

to grow rather widely spaced, and the relatively sparse foliage allows 
many patches of sunlight to reach the ground. The canopies of pines 
and oaks usually have fewer gaps. Measurements made in Illinois 
revealed that the portion of the forest floor exposed to direct sunlight 
was 84 per cent for poplar, 77 per cent for pine, and 35 per cent for 
oak. In elm-maple forests and in tropical rain forests the canopy 
typically is complete, with the result that none of the direct sunlight 
reaches the forest floor. Other measurements showed that the light 
on the forest floor which had passed through the leaves of the canopy 
was reduced in intensity to 3.5 per cent of its value above the tree 
tops in the oak forest; corresponding figures for elm-maple and rain 
forests were 0.4 per cent and 0.2 per cent, respectively. Thus less 
than 1 per cent of the light outside the forest reaches the ground if 
the canopy is complete. 

Different types of forests exhibit different seasonal influences on the 
light factor. Underneath a stand of pine trees the light is reduced 
by about the same amount throughout the year since the trees are 
evergreen. The illumination on the floor of a pine forest reaches a 
maximum in the early summer and a minimum in winter correspond- 
ing to the seasonal variation in the intensity of the incident sunlight. 
In a maple forest, however, a very different seasonal picture is pre- 
sented. During January, February, and March the plants and ani- 
mals living on the forest floor receive increasing amounts of light, but 
in April the leaves begin to appear and the intensity of light drops 
rapidly reaching a minimum in the middle of the summer. Then, as 
the leaves of the maple trees begin to curl and drop off a larger 
amount of light is allowed to filter through the forest canopy. The 
striking seasonal variation in the amount of ultraviolet radiation pene- 
trating different forest types is shown in Fig. 6.2. The changes in the 
total visible illumination arc of a corresponding nature (Park, 1931). 
Accordingly, if you were a beetle, or, if yon prefer, a lily growing in 
a maple forest, the brightest month of the year for you would be April 
and the darkest would be July. In this situation living organisms have 
modified the environment in respect to the light factor so that the 
seasons have been practically reversed. 

Duration and Amount of Light. The total amount of light received 
by an organism is determined both by the intensity of the light and 
the duration of the period of irradiation. The situation is similar to 
the exposure of a photographic plate. The amount of blackening of 
the plate is determined by the intensity of the light times the period 
of exposure. In natural habitats, the variation in the length of day 



Duration and Amount of Light 



189 



.080 



.070 




'"Jan. June Dec, 

FIG. 6.2. Mean monthly maximum intensities of solar ultraviolet radiation 
(2900-4000A) in forest communities of northern Indiana. ( Stroliecker, 1938.) 

often has a greater effect on the total amount of light received than do 
differences in the intensity of the sun at noon. 

On the equator the day is always 12 hours long, but in the temper- 
ate regions the day grows longer as spring progresses. This effect is 
accelerated at higher latitudes, and the day becomes 24 hours long 
during the summer in the polar regions. The day becomes cor- 
respondingly shorter after the summer solstice (Fig. 6.3). Up to 
moderately high latitudes the increase in length of day during summer 
has more effect on the total amount of light received per day than the 



190 



Light 



i . n .m .iv. v 4 vr vii viii ix. x xi xii i n m, iv v vi 



60 N 




40* 



60 S 



80* S 



FIG. 6.3. Variation in length of day at the indicated latitudes during the indicated 
months. Arrowheads indicate periods of decreasing daylength. The heavy line 
shows the course of zenith sun ( Baker, 1938. ) 







FIG. 6.4. Total solar radiation ( g-cal/day/cm 7 of horizontal surface) on June 21 
with cloudless sky. " ( Modified from Kimball, 1928. ) 



Extinction and Modification of Light 191 

reductions in solar intensity due to the greater angle of incidence. The 
greatest northward development of this influence is illustrated in Fig. 
6.4 in which the total solar radiation, determined by the combined ac- 
tion of intensity and duration, is plotted for June 21 under conditions 
of cloudless sky. At that period the amount of solar radiation received 
each day at 50 N latitude is considerably greater than that received 
at the equator, and the daily amount of light received in Scandinavia 
is roughly the same as that falling on tropical Africa. If you are 
amazed to find potatoes growing in Norway, or wheat being harvested 
in Alaska, don't forget that once the plants have begun to develop in 
the late spring, sufficient light exists in these latitudes for the plants 
to continue growing all day and a good part of the "night." 

The foregoing review of the circumstances of light on the earth's 
surface has shown that light is everywhere sufficient for life of some 
type at least for part of the year. Plants and animals exist from pole 
to pole. Mosses and lichens grow in abundance right up to the edge 
of the ice in the polar regions. On the other hand, light is nowhere 
too strong for animals and plants of some sort. Thus the whole range 
of light on the earth's surface is generally compatible with life. Of 
course, light may sometimes be too strong or too weak for individual 
species, and in these instances it controls growth and distribution. 

Light in Water 

The sunlight available for plants and animals in the aquatic en- 
vironment has entered the water from the air and hence has first been 
subjected to all the changes imposed upon it by the conditions above 
the surface. Ten per cent or more of the light is lost by reflection at 
the surface or in the special conditions just beneath the surface 
(Clarke, 1939). In addition, in passing downward, the light is fur- 
ther modified by the water medium in respect to intensity, spectral 
composition, angular distribution, and time distribution. 

Extinction and Modification of Light. The light factor in the water 
environment is subject to a number of variable influences that modify 
it profoundly. In order to consider the situation first in its simplest 
terms, imagine a lake from which we have pumped out all the natural 
water and refilled with distilled water. Our lake then contains water 
of uniform and maximum transparency from top to bottom. L Light is 
reduced in intensity both by absorption and by scattering, and the 
rate of reduction is measured as the extinction rate, although the term 
absorption rate is sometimes used loosely for the combined effect. 
Pure water causes the extinction of light at different rates in different 



192 



Light 



parts of the spectrum. Measurements of illumination made with a 
photometer placed in a watertight case and lowered into our imag- 
inary lake would reveal the changes presented in Fig. 6.5 At a depth 
of 70 m the blue component of sunlight has been reduced to about 
70 per cent of its intensity at the surface. At the same depth the 
yellow component of sunlight has been reduced to 6 per cent of its 
incident value. The orange and red components have been ex- 
tinguished very much more rapidly. At a depth of 4 meters, red 
light has already been diminished to about 1 per cent of its surface 
intensity. 



Percentage of incident light 

5 10 

TTT 



50 




FIG. 6,5. Reduction in intensity (logarithmic scale) of the color components of 

sunlight (indicated by initial letters) at increasing depths (linear scale) in a lake 

of optically pure water. (Clarke, 1939, AAAS Pub. No. 10.) 

Since we know that the daylight incident upon the surface of our 
lake differs markedly in intensity in different parts of the spectrum, 
we are dealing with the unequal extinction of an unequal spectrum. 
As shown in Fig. 6.6, the extinction by pure water of light at the two 
ends of the spectrum is much more rapid than in the middle of the 
spectrum. v The blue component of light is also the most penetrating 
in the clearest ocean and lake waters. v At depths of 100 m or more 



Extinction and Modification of Light 193 

the blue light becomes completely predominating;) this was observed 
directly by Beebe in his bathysphere dive off Bermuda. An observer 
looking straight down into the water from the deck of a ship in the 
clear tropical parts of the ocean sees the blue color resulting from this 
selective absorption. Since sunlight has been shorn progressively of 
its longer and shorter wavelengths, the only component remaining to 
be scattered upward again to the eye is the blue. Anyone who has 
seen the intense indigo of the tropical oceans will never forget it. 




- uv- 

3000 A 4000 5000 6000 7000 8000 

Wavelength 

FIG. 6.6. The spectral distribution of solar energy at the earth's surface and 

after modification by passage through the indicated meters of pure water. Similar 

light conditions would be found in the clearest ocean and lake waters. (Clarke 

1939, AAAS Publ No. 70.) 

The foregoing has shown that even pure water absorbs light at a 
very rapid rate compared to air and causes a profound change in 
spectral distribution. In the clearest parts of the ocean and in ex- 
ceptionally clear lakes the optical properties of the water are closely 
similar to those of pure water, Other natural waters contain sus- 
pended particles and dissolved material in sufficient quantities to 
cause a further reduction in transparency and a further alteration in 
spectral composition. 

Suspended material in the water includes living organisms that in- 
crease the extinction of the light and thus modify their own environ- 
ment in this respect. Illumination is reduced by beds of kelp along 



194 



Light 



the seacoast and by submerged or floating vascular plants along the 
shores of inland waters in much the same way that light is cut down 
by the larger vegetation on land. In the free water of ponds, lakes, 
and the oceans the phytoplankton is sometimes sufficiently abundant 
to produce a noticeable reduction of light. Plankton populations may 
cause an additional extinction of light indirectly by adding detritus or 
stains to the water after the organisms have died and disintegrated. 
A thick "bloom" of algae in a pond may thus reduce the light supply 
to such an extent as to curtail its own growth and that of other plants 
in the water layers beneath. 




B 







G Y 

Wavelength 

FIG. 6.7. Reduction in intensity and shift in spectral composition of light in 
heavily stained Rudolph Lake, Wisconsin. (Clarke, 1939, AAAS Publ. No. 10.) 

In temperate and coastal seas and in the majority of clear inland 
lakes fine particles or stains are present that tend to absorb or scatter 
the blue component of light more strongly than occurs in pure water. 
As a result the green component of sunlight is usually the most pene- 
trating in these situations and gives the water its characteristic emerald 
color. The organic stains occurring in some ponds and rivers absorb 
the shorter wavelengths so strongly that the red or orange components 
of sunlight become the most penetrating. In Rudolph Lake, Wiscon- 
sin, for example, the combined absorption of the water and of stains 
in the water causes not only a very rapid reduction in the light with 



Extinction and Modification of Light 195 

depth but also a shift in the position of the maximum light to the red 
region of the spectrum (Fig. 6.7). 

Since natural waters differ very greatly in transparency and in se- 
lective absorption, a study of the penetration of each part of the spec- 
trum in each body of water would have to be made for a complete 
description of the light conditions. An approximate comparison of 
the illumination in various natural waters can be made on the basis 
of the relative transparency to the central part of the spectrum. 
Sample values are given in Table 14 for representative bodies of 

TABLE 14 

TRANSPARENCY OF WATER TO CENTRAL PART OF VISIBLE SPECTRUM 

(5000-0000 A) 

Extinction Depth for \% of Secchi Disc 

per Meter (<*;.) A:* Surface Light Depth f 

Distilled water 3.8 . 039 118 m 44 m 



Caribbean Sea 


4.1 


0.041 


110 


41 


Continental slope 


7.2 


0.072 


60 


24 


Gulf of Maine 


10.0 


0.10 


42 


17 


Woods Hole Harbor 


SO . 


0.30 


1(3 





Crater Lake, Oregon 





. 00 


77 


28 


Crystal Lake, Wis. 


15 


0.10 


28 


11 


Trout Lake, Wis. 


33 


0.40 


11 


4 


Midge Lake, Wis. 


78 


1.5 


3 


1 


* Extinction coefficient 


, k. 












* _ p -kL 










h 







where /o = initial intensity. 

7 = intensity at depth, L, in meters. 
f = 2 7 

c/ /c. i . 

t Depth at which a 20-cm white disc disappears when lowered from the 
surface. 

water. Since the great differences in the extinction of light per meter 
have a cumulative effect, an even greater contrast is presented in the 
depths at which a given fraction of the light is found. The depth at 
which the light intensity is reduced to 1 per cent of its surface value 
is of particular importance because it represents the approximate 
lower limit for plants, as will be discussed later. In general the table 
and the representative curves in Fig. 6.8 show the profound variation 
in the intensity of light available for organisms at increasing depths 
in different natural waters. 



196 



Light 



Another way in which light is changed by water is in its angular 
distribution. The direct beam of light from the sun is bent by re- 
fraction toward the perpendicular at the water surface. Since waves 
or ripples almost always exist in natural situations, the light from the 
sun tends to be broken up to a considerable extent in passing through 
the surface. Within the water itself further diffusion is produced by 
scattering. When larger amounts of suspended materials are present, 
scattering proceeds at a very rapid rate, with the result that measur- 



0.1 



Percentage of surface light 
0.5 1.0 5 10 

INN I 




FIG. 6.8. Comparison of the transparencies of representative natural waters. 
Curves based on the average extinction rate for the yellow-green (5000-6000A) 
component of daylight. (1) Midge Lake, Wis., (2) Trout Lake, Wis., (3) Gun- 
flint Lake, Minn., (4) Woods Hole Harbor, (5) Thatcher Pass, San Juan Islands, 
and Buzzards Bay, Mass., (6) Vineyard Sound, (7) Baltic Sea, (8) Crystal Lake, 
Wis., (9) English Channel, (10) Gulf of Maine, (11) off Vancouver Island, and 
off Nantucket Shoals, (12) Crater Lake, Ore., (13) Gulf Stream, (14) Caribbean 
Sea, ( 15) distilled water. (Clarke, 1939, AAAS Publ No. 10.) 



Changes in Transparency 197 

able light is passing in every direction. In the upper strata the light 
passing in the direction of the retracted rays from the sun is the 
strongest. With increasing depth the direction of strongest light in- 
tensity tends to move toward the vertical, and a limiting pattern of 
diffusion is established (Jerlov, 1951). Thus, although a variable de- 
gree of diffusion occurs, the directional character of the light in the 
water is never entirely lost. 

Changes in Transparency. Parts of the ocean and of lakes that 
are well stirred exhibit a uniform transparency from the surface down- 
war6\ but, when strong density gradients exist, considerable changes 
in transparency may occur with depth. Furthermore, the transpar- 
ency of coastal marine areas and of inland water bodies often changes 
profoundly from season to season, owing to differences in amount of 
stirring, in the discharge of muddy rivers, and in the growth of plank- 
ton. Such variations in transparency are added to the seasonal 
changes in the intensity of the light received at the surface and in the 
length of the day. The* combined effect of these influences results 
in surprisingly great fluctuations in the light factor at subsurface 
levels. 

Observations made off the coast of Massachusetts illustrate the mag- 
nitude of the foregoing seasonal influences (Clarke, 1938). In this 
region the maximum hourly intensity of light received at the surface 
during the summer averaged about twice that received during the 
winter period. The average total solar radiation received per day 
in summer was about four times as great as that received during the 
winter. Significant departures from the average for periods up to 
several weeks occurred, and such variations are undoubtedly im- 
portant in causing changes in the response to the light factor from 
year to year. Values for individual days may also depart markedly 
from the average. During the investigation referred to above the 
maximum light received on the brightest day of summer was forty 
times greater than that received during the dullest day in winter. 
In the course of the same year the extinction coefficient of the water 
varied fourfold. Since the effect of extinction is cumulative, the il- 
lumination in the subsurface layers varies more because of changes 
in transparency than because of seasonal changes in radiation reach- 
ing the surface. By combining the two effects it was found that for 
an organism living at a depth of only 30 in the minimum hourly 
light intensity was 7000 times greater and total daily radiation was 
10,000 times greater in May, when high incident light was combined 
with high transparency, than in December, when both incident il- 
lumination and transparency were low. The magnitude of seasonal 



198 Light 

changes in illumination in the aquatic environment are therefore fre- 
quently very much greater than in the terrestrial environment. Any 
water body that displays a much higher transparency in winter than 
in summer will have less light available during the summer in its 
deeper layers and thus suffer a reversal of the usual seasonal change 
in the light factor. The foregoing instances are sufficient to show 
that in the aquatic environment light becomes profoundly altered 
quantitatively and qualitatively and that its changes may go far be- 
yond those experienced by plants and animals on land. 

BIOLOGICAL EFFECTS OF LIGHT 

Before concerning ourselves with the influence of the light factor 
on growth, reproduction, locomotion, and other activities of the or- 
ganism as a whole, we shall consider certain general effects of radia- 
tion as it strikes the surface of the plant or animal. In addition to 
heating the tissues, the absorbed radiation affects biological processes 
in the exposed tissues, including particularly their pigmentation. 

General Effects 

Among green plants light is required for the production of chloro- 
phyll in the chloroplasts. Plants germinated under insufficient il- 
lumination will not develop their normal green color. Normal plants 
become etiolated in the absence of light; that is, they lose their pig- 
ment and develop abnormal form. On the other hand, excessive il- 
lumination causes the destruction of chlorophyll. In some plants ex- 
cessive absorption of light by the deeper tissues is prevented by the 
screening action of thickened chloroplasts or of increased numbers of 
chloroplasts near the surface. In some species when the light be- 
comes too bright, the chloroplasts line up one behind another so that 
a larger proportion of the radiation passes through the leaf between 
the chloroplasts. When the light becomes weak, the chloroplasts 
spread out and absorb a maximum percentage of the incident illu- 
mination. 

Light may influence th(^ pigmentation of animals in several ways. 
Skin color may be indirectly affected by light through the mediation 
of the eyes or other receptors. In other instances differences in color 
may have arisen as a result of selective survival. The abundance of 
pigment exposed in the chromatophores is sometimes directly con- 
trolled by the intensity of the light received. The characteristic lack 
of pigment in cave animals is associated with darkness, and certain 



Protective Coloration 199 

aquatic forms have been shown to lose their color when removed from 
light. Blind cave amphibians and fishes with little or no color have 
been found to develop abundant pigment in the skin after exposure 
to normal daylight (Rasquin, 1947). Intense radiation may be harm- 
ful because of undue heating or evaporation, because of the lethal 
action of the ultraviolet component, or in other ways. 

The excessive absorption of light by animal tissues must also be 
avoided. Most animals simply move into the shade, burrow into 
the ground, or descend to deeper levels in the water. The develop- 
ment of a relatively transparent body would appear also to help deal 
with this problem. Since only the radiation that is actually absorbed 
can be effective, the highly transparent tissues of many types of plank- 
ton can retain but little light energy. This fact may enable some 
planktonic animals to endure higher light intensities in the surface 
waters than would otherwise be possible. As we shall see, many 
species of plankton are extremely sensitive to light. In other kinds 
of animals evolutionary development has gone in the opposite direc- 
tion with the production in the skin of abundant pigment which pro- 
tects the deeper tissues. Anyone who has lain too long on the beach 
in June has been painfully aware of the harmful effect of excessive 
radiation on animal tissue and is familiar with the deposition of pig- 
ment in the skin of man to produce tan. In addition, transparency or 
pigment patterns also serve to render animals less conspicuous. 

Protective Coloration. The pigmentation of a great many animals 
forms a coloration that appears to afford protection from enemies. 
One very common type of protective coloration is a simple matching 
of the background in respect to color and pattern. A quail squatting 
in the grass or a bittern standing motionless among the swamp reeds 
is exceedingly difficult to distinguish from its surroundings, as is a 
moth on the bark of a tree or a katydid among green leaves. Many 
other instances of remarkably close resemblance of birds, insects, and 
other animals to their background will occur to the reader. A second 
common type of protection, often combined with the first, is oblitera- 
tive shading in which the bird, mammal, or fish displays darker pig- 
mentation on its back and lighter color underneath. This difference 
tends to counteract the stronger illumination received from above 
with the result that the animal blends with its background. 

The protective aspect of other color patterns may result chiefly from 
the confusion of the enemy by the disruption of the animal's usual 
outline. Birds with necks or heads of contrasting colors or tropical 
fish with strong transverse stripes are not easily recognized by man 
under certain conditions of illumination because of the unexpected 



200 Light 

pattern and presumably are not easily recognized by their natural 
enemies. Any such visual deception will, of course, not affect preda- 
tors that locate their prey exclusively by smell, sound, or other meth- 
ods. Some species when disturbed produce a flash of some brightly 
colored or contrasting part that is concealed when the animals are 
quiet, and the startling effect may serve to distract or to frighten off 
pursuers. 

Another type of protective resemblance is mimicry a phenomenon 
that is particularly well developed among the insects. Here one 
species closely resembles in color and form another totally unrelated 
species sometimes in an entirely different order. The mimic is be- 
lieved to derive protection from the fact that it is mistaken by preda- 
tors for the species it resembles. If the model species is distasteful or 
harmful as a food organism and hence is avoided by the predators, 
the mimic may also escape unmolested. Extraordinarily close 
mimicry certainly exists between insect species, but the reality of the 
benefit of the mimicry, its mode of operation, and its evolutionary 
origin are controversial subjects which still await conclusive investi- 
gation. For a further discussion of the far-reaching problems of pro- 
tective coloration the reader may consult more extensive treatments 
of the subject such as that by Cott (1940). 

Some animals are able to change their color or pattern sometimes 
within a matter of minutes or even seconds, Such changes in ap- 
pearance occur characteristically as adaptations to the background, 
and are found among reptiles, amphibians, fishes, crustaceans, insects, 
cephalopods, and other invertebrates. The mechanisms by which 
color changes are brought about have been summarized by Prosser 
(1950, Ch. 21). In many instances they involve the nature of the 
light received from the background through the eyes, but in other in- 
stances, they are activated by direct radiation. A flounder changes 
its general color tone and also the pattern of the black and white 
patches of its skin as it moves from one type of bottom to another. 
Such changes in coloration tending to match the background furnish 
an obvious advantage in concealment (Summer, 1935). In other in- 
stances color changes serve as protection from high illumination, take 
part in thermoregulation, or are associated with breeding, as in cer- 
tain lizards, fishes, and squids. The seasonal color changes of the 
varying hare, weasel, and ptarmigan from brown in summer to white 
in winter are obviously related to the conspicuousness of such animals 
against bare ground or snow-covered landscape. 

The different, and usually more brilliant, coloration of the males 
of many birds and of some other animals is familiar to everyone. In 



Protective Coloration 201 

attempting to explain the evolutionary origin of this sex difference, 
Darwin drew attention to the courtship antics of peacocks, pheasants, 
and other birds, in which the males appear to vie with one another 
in showing off their plumage. Darwin proposed that the special 
plumage of the male evolved after many generations in which the 
females selected and mated with the "most beautiful" birds. Since 
we have no reason to suppose that birds judge the attractiveness of 
male suitors on the same basis as we would, this explanation seems 
unsatisfactory. As yet no convincing demonstration has been made 
of any basis for selection or of any other method by which the elab- 
orate decoration of the male may have arisen. 

The duller coloration characteristic of the female is undoubtedly 
related to her greater need for concealment while brooding the eggs. 
The striking coloration of the males of many species can rarely have 
any protective value for the male himself, although his conspicuous- 
ness might draw attention away from the female on the nest. In some 
birds, such as Wilson's phalarope, the tables are turned, for the fe- 
males are brightly colored and the drab males do the housework of 
incubating the eggs. Difference in appearance may play a useful 
part in aiding sex recognition. The brilliant breeding plumage of 
the male is often replaced by a duller garb during the winter season. 
The length of day has been shown to influence breeding, migration, 
and color change in many birds and mammals. The light factor may 
thus be involved in coloration through its effect on reproductive ac- 
tivities as well as through its role in protective resemblance, and the 
two may be interrelated. 

Mention was made in earlier chapters that desert animals charac- 
teristically display a pale coloration in contrast to the darker hue of 
the inhabitants of humid regions. Although temperature, moisture, 
and light may directly affect the general color of terrestrial animals 
under some circumstances, these factors often act indirectly through 
their influence on the color of the ground in relation to the conceal- 
ment of the animals. Evidence indicates that pale or dark coloration 
has evolved in many species and races as a result of selective survival 
as influenced by their conspicuousness against the background. Se- 
lection would thus account for the occurrence of a white mouse 
(Perognathus apache) inhabiting an area of white gypsum sands in 
New Mexico and of a black mouse (P. intermedius) living on an ad- 
joining area of black lava (Benson, 1933). The possibility that cli- 
mate sometimes exerts a direct effect, however, is suggested by the 
fact that nocturnal animals in the desert have the same pale coloration 



202 Light 

as those which are abroad during the daylight hours when light would 
appear to be much more important. 

' It is a mistake to assume that the pigment developed by an organism 
Necessarily plays a critical role in its present relationships. In some 
instances the pigment; may be primarily a by-product of metabolism 
without any ecological significance. The red color of the deep-sea 
shrimp is a case in point. Since there is no red light deep in the 
water, the shrimp must appear as black as the deep-sea fish which 
share the same habitat. Similarly, it is unnecessary to assume that 
the coloration of the scarlet tanager, the oriole, and other birds with 
brilliant plumage provides any protective resemblance. Certain 
writers have practically turned themselves inside out in attempts to 
find a protective function for all bright colors. No one in his right 
mind would try to claim that Chromodoris, a bright blue nudibranch 
with orange and gold spots, was protectively colored as it creeps over 
the gray rocks and brown seaweeds of the tidal zone of southern 
California. In such instances either the bright colors do not attract 
enemies, or the organism survives in spite of being conspicuous be- 
cause of the possession of other, sufficiently advantageous attributes. 

Activity and Vision 

Photokinesis. Light controls the locomotory activity of many of the 
lower organisms by a direct action upon their speed of locomotion 
a phenomenon known as photokinesis. The magnitude of this reac- 
tion among animals without eyes was well illustrated by a laboratory 
test on the larvae of the mussel crab, Pinnotheres maculatus (Welsh, 
1932). These animals swam the length of a 29-cm trough toward a 
light of 0.5 meter-candle in 34 sec. As the light intensity was in- 
creased, their swimming rate was accelerated regularly until at 46 
meter-candles they made the trip in 17 sec. Many other animals 
show a similar increased activity under increased illumination whether 
or not they are oriented to the light. This simple direct relationship 
is of profound importance in the lives of many of the aquatic inverte- 
brates and of the smaller terrestrial forms including insects. 

Vision. When activity is controlled by light among higher forms 
it is usually through vision. We should inquire as to the circum- 
stances of illumination under which vision is possible. On land day- 
light is everywhere sufficiently strong at some period for the vision of 
those animals that possess eyes. In the water because of the rapid 
rate at which light is reduced in intensity we may suspect that depths 
would soon be reached at which vision is no longer possible. Let us 



Vision 203 

inquire what is the minimum illumination for aquatic animals to see 
sufficiently to feed, to find their mqtes, or to avoid dangers. Interest 
also centers on the maximum depths in the aquatic environment at 
which responses to day and night still exist. 

A laboratory test can be set up in which the intensity of light is 
varied over a considerable range with relative ease. But it is not so 
easy to ask a fish whether or not he can see small objects at least 
it is not so easy to get him to reply. An answer was obtained, how- 
ever, from the fresh-water sunfish, Lepomis, by using the response of 
the fish to background motion. If this type of fish is placed in a 
glass cylinder with a surrounding screen made of bars and spaces, the 
fish responds by a turning motion if it sees the screen rotate. It is 
through this kind of reaction that many fish maintain their position 
in a stream. Reactions of this sort are involved in rheotaxis, dis- 
cussed in an earlier chapter. By reducing the light intensity in an 
experiment of this type until the sunfish no longer responded, the 
minimum illumination under which the fish could see small objects 
similar in size to the bars of the screen, was found to be one ten- 
billionth (or 10 ln ) of the value of full sunlight (Grundfest, 1932). 
The threshold sensitivity of the human eye is similar, and the value 
for other vertebrates is probably generally of the same order of 
magnitude. 

Assuming that the threshold illumination for other species of fish 
is similar to Lepomis, determinations were made from transparency 
measurements of the approximate depths at which this minimum 
intensity of light would occur under different circumstances, and 
hence the maximum depths at which vision would probably be pos- 
sible, although other optical conditions such as color and contrast with 
background would have to be taken into account (Clarke, 1936). 
In most lakes except the most turbid and in typical coastal areas 
vision would appear to be possible for fish similar to Lepomis at all 
levels right down to the bottom. In the open ocean beyond the mar- 
gins of the continental shelf the water is far deeper than this maxi- 
mum depth for vision. If deep-sea fishes can see as well in blue 
light as the sunfish tested could see in green light, then vision would 
be possible for them at depths of more than 700 m in the clear 
tropical ocean. 

Many aquatic animals may show some activity response to the 
increase or decrease of light at still lower intensities. Studies of the 
diurnal vertical migration of zooplankton indicate that certain species 
may react to light from the surface at 800 m and possibly at 1000 m, 
or more than half a mile down. This depth probably represents the 



204 Light 

biological zero for the response to daylight penetrating from the sur- 
face. Below this level day and night no longer exist, and no seasonal 
change in illumination can be detected. Since the average depth 
of the ocean is about 4000 m, it is clear that more than three-fourths 
of the volume of this environment is devoid of any influence of day- 
light. The whole of the deep sea is by no means completely dark, 
however. Light of biological origin occurs irregularly through the 
marine environment, and bioluminescent organisms are sometimes 
very abundant in the sea. 

Bioluminescence. Our discussion up to this point has dealt pri- 
marily with light emanating directly or indirectly from the sun, but 
light of biological origin, known as bioluminescence y or popularly as 
"phosphorescence," has ecological significance under certain circum- 
stances. As mentioned above, bioluminescence is the only source 
of light in the deep sea. Near the surface of the ocean and on land 
it is frequently prominent during the night, but it rarely occurs in 
fresh water. Luminescence is produced by members of various taxo- 
nomic groups scattered through the animal kingdom including in the 
sea certain fishes, crustaceans, coelcnterates, and many other in- 
vertebrates, and on land particularly the insects. Fungi and such 
microorganisms as the dinoflagellates and many groups of bacteria 
also are luminescent. For a complete summary of the occurrence 
and physiology of bioluminescence the reader should refer to the 
book on the subject by Harvey (1952). 

The very considerable amount of illumination that can be provided 
by luminescent organisms may be appreciated by anyone who catches 
some fireflies and brings them into a darkened room. A few fireflies 
in a bottle will provide sufficient light for reading newsprint. The 
"phosphorescence" of the sea sometimes is almost dazzingly brilliant 
on a dark night, at which time waves and the wake of a boat appear 
like "burning water." The bodies of fish and other organisms swim- 
ming through the water are outlined by millions of tiny lights. This 
illumination is caused by the luminescent discharge of the plankton 
organisms. Measurements have shown that the intensity of light 
emitted by the surface waters of the sea may be one thousand times 
greater than the threshold intensity for the vision of man. 

Bioluminescence is thus sufficiently strong to evoke reactions under 
a variety of circumstances. Although this living light may serve no 
useful purpose in some situations, in other instances it probably 
fulfills one or more of the following functions: (1) illumination, (2) 
recognition, (3) lure, and (4) warning. 

The luminescence of deep sea animals undoubtedly provides a use- 



Biolwn inescence 205 

ful amount of illumination for the individuals producing it as well 
as for other inhabitants in the immediate neighborhood. In shallow 
water and on land during the night animals may similarly use their 
photophores as lanterns. The employment of luminescence in recog- 
nition of one sex by another is clearly exemplified by the firefly. 
Among the American Lampyridac each species has a characteristic 
code of flashing by which the females can recognize males of the 
same species and distinguish them from males of other species. After 
dark the female climbs up on a blade of grass and when a flying male 
signals in her vicinity she attracts him to herself by recognizable 
flashes in response (Harvey, 1952, Ch. 13). 

In the abyssal depths of the sea, fish can perhaps locate each other 
by recognizing the pattern of lights presented by the luminescent 
organs. It has also been suggested that squid are able to keep to- 
gether in a school during the dark hours of the night by means of 
their characteristic flashing. 

Animals may lure prey by means of their luminescent organs. 
Since many small fish and planktonic invertebrates are attracted to 
light, these animals would be expected to move toward the lumi- 
nescent organs of predatory fish, many of which are located near the 
jaws or even on filaments dangled in front of the mouth. On the 
other hand, the sudden flash of luminescent organs may act as a 
warning to scare off predators. Certain deep-sea shrimps produce 
a luminescent secretion that may be discharged into the water. This 
is an interesting counterpart to the sepia produced by squid and 
cuttlefish in shallow water. When the latter are attacked, they can 
discharge the black secretion into the water and escape from their 
enemies in the "smoke screen'* thus produced. In the inky blackness 
of the deep sea the shrimps that produce a luminous discharge when 
attacked may be able to escape in a "cloud of light" (Fig. 6.9)-~or 
these sudden emissions may act simply by distracting or frightening 
the enemy. 

The possession of luminescence may be a definite disadvantage for 
some organisms if it gives away their presence to enemies. For other 
species the emission of light may be an accidental by-product of 
metabolism and hold no ecological significance whatsoever. We can- 
not imagine, for example, any possible benefit for bacteria that could 
be derived from their production of luminescence. 

A correlation has been thought to exist in the marine environment 
between the occurrence of eyes, the type of coloration, and the pres- 
ence of daylight or luminescence. Eyes are well developed among 
animals inhabiting the upper layer of the ocean, and luminescent 



206 Light 

organs occur in widely different groups. At somewhat deeper levels 
the eyes of fish tend to be enlarged or "telescopic" and in some species 
directed upwards. At greater depths eyes are often degenerate or 
entirely absent. Eyes would perhaps be of little use here since 
both daylight and animal light produce very meager illumination. 




N.C.S. Copyright, 1934 

FIG. 6.9, Drawing to illustrate the appearance of the photophores of the deep 

sea fish Photostomias guernei and of the luminous discharge of the shrimp 

Acanthephyra purpurca in the inky blackness of the ocean abyss. 



Orientation 207 

In the ocean abyss by contrast, many fish possess well-developed 
eyes and the bottom-living organisms produce abundant luminescence 
(Sverdrup et al., 1942). 

In caves no luminescence is produced by the aquatic inhabitants 
and only one terrestrial form-the New Zealand glowworm (Arach- 
nocampa lwninosa)-is known to be luminescent. The fauna of caves 
that are completely cut off from daylight consists typically of species 
that are blind or have degenerate eyes. Cave animals usually have 
little pigment and contrast strongly with the jet black or dark red 
deep-sea animals. The heavy pigmentation of the latter may perhaps 
be explained as a protective adaptation serving to reduce the reflec- 
tion of luminescent light from their surfaces and hence the chance 
of detection. Since cave waters are completely dark, the whitish 
coloration of the inhabitants would be of no disadvantage on this 
score. 

Orientation 

Light often plays a significant role in orienting the growth or 
locomotion of plants and animals. Since orientation to light is often 
associated with reactions to other factors in the environment, such as 
gravity, we shall first consider the subject in general terms. Orienta- 
tion is brought about either by the differential growth or movement 
of parts of the organism or by a change in the direction of locomotion 
of the whole organism. The question of why plants and animals grow 
or move in the directions they do has occupied the attention of in- 
vestigators for a long time. Many of the theories and terms used have 
been in conflict, and for a more extended discussion reference should 
be made to Fraenkel and Gunn (1940) and to Griffin (1953). 

The term tropism is best used for orientation by growth or turgor 
movements as exhibited by sessile forms. These forms are usually 
plants, but essentially the same type of orientation is exhibited by 
many sessile animals such as the hydroids. If the orientation is to 
gravity, the term geotropism is used. If it is to light, the growth 
movement is referred to as phototropism, and other prefixes are used 
for other orienting factors. OfTthe other hand, the orientation of 
the locomotion of motile organisms is best referred to as a taxis, 
although it is sometimes also called a tropism. Here the forms in- 
volved are usually animals, although motile plants such as the green 
flagellates and motile plant gametes or zoospores are included. Ap- 
plying suitable prefixes, we obtain the terms geotaxis, phototaxis, etc., 
for orientation by the corresponding factors. Orientation in the direc- 
tion of an orienting force is referred to as a positive tropism or 



208 



Light 



taxis; orientation in the opposition direction is a negative tropism or 
taxis, Occasionally a transverse tropism or taxis is displayed. 

Control of speed of locomotion by the intensity of the factor is 
termed a kinesis. Gravity never changes significantly in intensity, 
but the speed of swimming, flying, or creeping is influenced by altera- 
tions in the strength of other factors; the common occurrence of 
photokinesis has already been mentioned. 

The position of the main axis of the body is the primary orientation 
of the plant or animal. This is usually determined by gravity, as is 
seen in the upward growth (negative geotropism) of the shoot and 
the downward growth (positive geotropism) of the root of a plant 
seedling (Fig. 6.10). In the aquatic environment the buoyant action 






FIG. 6.10. Negative geotropism of shoots and positive geotropism of roots of four 

kernels of maize that have germinated in different positions. ( By permission from 

Botany by Sinnott, 1929, McGraw-Hill Book Co. ) 

of water often reduces the effect of gravity so that primary orienta- 
tion is to light or to current. When the source of illumination is 
from the side, the primary orientation of a land plant will be a 
compromise between the influence of gravity and that of light. 

The primary orientation of motile organisms is also usually to 
gravity, and a walking or a flying animal continually adjusts its posi- 
tion to it. However, a fly on the ceiling or on a wall is quite uncon- 
cerned as to which side up it is, since it is oriented primarily in rela- 
tion to the surface upon which its feet rest. Sometimes orientation 
is brought about passively by the resistance of the appendages as 



Orientation 



209 



gravity draws the organism through the medium, as is seen in a 
"floating" butterfly or in slowly sinking plankton. 

Superimposed upon the primary orientation, many secondary re- 
sponses to orienting influences take place. The tips of green plants 
grow toward the light and hence exhibit positive phototropism. The 
leaves are oriented generally at right angles to the incident radiation, 
thus receiving maximum illumination, and they sometimes form a 
symmetrical pattern (Fig. 6.11) and sometimes a mosaic, as is dis- 
played by ivy leaves on a wall. 







FIG. 6,11. Alternating position and horizontal orientation of leaves on shoot of 

Norway maple, resulting in maximum exposure to sunlight. (Shipley, 1925, after 

Kerner, Copyright, Cambridge Univ. Press.) 

In such species as the sunflower the top portion of the plant or its 
leaves are turned by turgor changes during the course of the day, 
keeping always in the direction of the sun. In regions where light 
and heat from the sun are excessive, the leaves of some plants, such 
as the compass plant, Silphium laciniatum, are oriented so as to 
present their edges toward the sun. These examples will suffice to 



210 



Light 



illustrate the turning reaction of phototropism by actual growth or by 
turgor movements. 

Phototaxis, geotaxis, and other tactic reactions involve the orienta- 
tion of locomotion toward or away from light, gravity, or some other 
source of stimulation in the environment. In many instances the turn- 
ing of the moving organism appears to be due to the unequal stimu- 
lation of symmetrical sense receptors that control the tonus of the 
body, leg, or wing musculature. When the animal moves, it is caused 
to turn by the unequal posture of its locomotory apparatus. For 
example, in a positively phototactic animal such as the swimming 
insect Ranatra, if the left eye receives more light than the right eye, 
the legs on the left side of the body will be more strongly flexed, 
whereas the legs on the right side of the body will be more greatly 





FIG. 6.12. (Upper) Symmetrical swimming position of positively phototactic 
Ranatra moving toward a light source in front of it. (Lower) Position of normal 
animal turning toward a light source at its left. This position will be produced 
permanently if the right eye is removed; if both eyes are removed, the animal's 
position becomes symmetrical again. (Crozier, 1929, Copyright, Clark Univ. 

Press.) 



Orientation 211 

extended and will make longer sweeps. As the animal swims in this 
posture, it will tend to turn toward the source of light. Turning will 
continue until the amount of light received in the two eyes is equal, 
whereupon the legs will function symmetrically and the animal will 
swim straight ahead (Fig. 6.12). In many such instances the turn- 
ing of the animal appears to be produced quite mechanically, and 
such reactions were referred to as "forced movements" by Jacques 
Loeb (1918) who originated the theory of orientation just described. 
In other instances orientation appears to come about by a trial-and- 
error procedure in which the animal changes its direction when it 
encounters unfavorable conditions and tries other directions until it 
finds a course in which conditions no longer stimulate it adversely. 




Fie. 6.13. Arrangement of experiment to measure the geotactie orientation at 
angle of eaterpillar Malacosoma placed on plane inclined at angle a. Lateral 
movement of head indicated by h and ti. (Crozier, 1929, Copyright, Clark Univ. 

Press.) 

Whether one adopts or rejects a mechanistic viewpoint in inter- 
preting the reactions of organisms, the fact is that tropistic and 
tactic responses of plants and lower animals play a major role in 
their lives under natural conditions, and their orientation often ap- 
pears to be rather rigidly controlled. An example of the mechanical 
way in which an orienting force may act is provided by an experi- 
ment involving the negative geotaxis of the tent caterpillar, Mala- 
cosoma. If this caterpillar is placed horizontally on a steeply sloping 
surface, an unequal stimulus of the proprioceptors located within 
the two sides of the body will result, owing to the animal's weight. 
A differential response in the tonus of the body muscles will then be 
produced, with the result that, as the animal creeps, it will turn to 
move up the slope (Fig. 6.13). As the caterpillar turns from its 
initial horizontal position toward the vertical, the difference in stimu- 
lation of the two sides of the body becomes progressively less, and 
eventually reaches a threshold beyond which no further turning is 
elicited. If the plane upon which the caterpillar is creeping is only 



212 Light 

slightly inclined (angle a), threshold discrimination is reached after 
a smaller amount of turning, or at a smaller final angle of orientation 
(6). When the plane is steeper, the animal continues turning to a 
greater angle of orientation. The result of a series of trials made 
with varying angles indicates the mechanical nature of the taxis and 
the quantitatively exact way in which the response of the animal is 
controlled by the magnitude of the orienting stimulus (Fig. 6.14). 



90 



s? 





S 



80 



70 



60 



50 



40 



30 



7 



10 



7 



20 



30 



90 



80 



60 



70 



50 



40 



30 



80 



90 



40 50 60 70 
Inclination of surface, a, degrees 

FIG. 6.14. Relation between the angle of inclination of plane (<*) and the mean 
angle of orientation (0) of caterpillar creeping upon it. Open circles are means 
for one individual; black circles are means for all individuals tested. The curve 
is that for A0/A log sin <* constant. ( Crozier, 1929, Copyright, Clark Univ. 

Press. ) 

Orientation of growth and locomotion in relation to light and other 
factors plays a profoundly important role in the lives of plants and 
lower animals. The responses of all species that survive must ob- 
viously generally bring the organism into favorable surroundings, but 
lack of flexibility in these reactions often produces harmful results 
under extreme or unusual conditions. The rigidity of the tactic 
responses of the tent caterpillar is often seen to lead to its destruction. 
When the animals emerge from their nests in the crotches of trees, 
they crawl upward in response to their negative geotaxis to the tips 



Orientation 



213 



of the branches where no doubt the juiciest leaves are found. The 
caterpillars later may return to the nest by following the silk threads 
they have left behind them on their upward journey, but they will 
not ordinarily move further downward over fresh surfaces. The two 
wild cherry trees shown in Fig. 6.15 have been completely defoliated 




FIG. 6.15. Wild cherry trees in southern Rhode Island defoliated by tent cater- 
pillars (Malacosoma), whose nests are seen in the forks of the branches. Neigh- 
boring oak trees were not attacked. 

by tent caterpillars. Following the consumption of all the leaves in 
their tree, many caterpillars starve, or die of disease (Craighead, 
1950) but others crawl or fall to the ground and eventually find their 
way to another tree. For some reason, as yet unknown, this species 
does not ordinarily eat oak leaves, and in the area photographed the 
oak trees only a meter or so away were not attacked. As the time for 
pupation approaches, negative geotaxis becomes strong and the ani- 
mals tend to climb up any vertical object encountered. In a neighbor- 
ing area a series of fence posts was found, on the top of each of which 
was a seething mass of tent caterpillars that had gathered there as a 
result of the reactions just described. Since the animals were slaves to 
their geotaxis, they remained on the fence posts, and, finding nothing 
else to eat, they proceeded to devour each other. 

When an animal moves vertically under normal illumination from 
above, it is often difficult to ascertain whether the animal is reacting 
primarily to gravity or to light; that is, if it moves upward, it may be 



214 



Light 



displaying a negative geotaxis, a positive phototaxis, or both. Ex- 
periments can be devised that distinguish between the responses 
of which the animal is capable. An example of such an experiment is 
given in Fig. 6.16 in which the response of a negatively geotactic 
and a negatively phototactic Agriolimax is shown. When light is 
allowed to act at right angles to gravity, this lowly garden slug can 
resolve the forces with more alacrity than is displayed by some col- 
lege students! The first trial indicated in the diagram was made 
when the animal was dark-adapted and its reaction to light was 
stronger than that to gravity. A decrease in the strength of photo- 
taxis as the animal became light-adapted is indicated by the change 
in the angle of orientation in successive trials. 



\ 



V I 




\ 




FIG. 6.16. (Upper) Orientation of negatively geotactic and negatively photo- 
tactic Agriolimax on a vertical plane with light from the right. L = phototactic 
vector, g geotactic vector, ~ angle of orientation. (Lower] Successive trials 
at 1 -minute intervals made by initially dark-adapted Agriolimax on a vertical 
plane. (Crozier, 1929, Copyright, Clark Univ. Press.) 

Other complications occur in the orientation to light. Diffuse 
light has been shown to exert an effect upon planktonic copepods that 
is different from that produced by light from a single source. Many 
plankters are found to be positively phototactic to an electric light, 



Orientation 215 

but these animals do not generally swim upwards to the surface of 
the water toward the sun since the diffuse side illumination in the 
ocean inhibits the reaction to a single source (Schallek, 1943). 

The familiar "bee-line" course which the honeybee takes from its 
hive to a source of food is determined as a definite angle to the direc- 
tion of the sun. The bee does not fly directly toward or away from 
the sun, but orients at an angle to the changing azimuth of the sun. 
Upon returning to the hive, the bee communicates to its fellow work- 
ers by a special "wagging dance" both the approximate distance and 
the direction relative to the sun of the food that it has discovered. 
Still more remarkable is the fact that the worker bee is not required 
to see the sun directly but can be oriented by a small patch of blue 
sky. The bee's compound eye has been shown to be sensitive to the 
angle of polarization of sky light, and the bee apparently uses this 
information in combination with other orienting forces to determine 
the proper line of flight when the sun is obscured. The bee usually 
restricts its visits to one species of plant at a time a fact that is of 
obvious advantage both to the bee's efficiency and to the plant's 
successful pollination. Flower species are recognized by shape, scent, 
and color; the bee's eye can distinguish yellow, blue-green, blue, and 
ultraviolet, but it is blind to red ( von Frisch, 1950 ) . 

The ability of homing and migrating birds and fish to find their 
way over long distances is perhaps even more amazing. The precise 
method of navigation used by such far-ranging vertebrates has mysti- 
fied observers for generations. Although birds may use landmarks, 
persisting cloud formations, prevailing winds, and other ecological 
cues to some extent, these possibilities do not appear to explain all 
instances (Griffin, 1952). Evidence has been obtained that certain 
species of birds may use the direction of sunlight as a means of 
orientation and that the birds are able to allow for the change in the 
sun's position during the day (Kramer, 1952). 

Under natural conditions the direction of growth or of locomotion 
of plants or of animals is usually determined by several different in- 
fluences acting simultaneously. In addition to such common orient- 
ing factors as light, gravity, temperature, and moisture, certain species 
may be guided by sound or other types of vibration, by scent or other 
chemical sense, or by other special reactions to environmental 
stimuli. We know that tropistic and tactic responses play a very large 
role in the lives of most lower organisms in nature. Reactions to the 
direction and intensity of the orienting influences of the habitat are 
responsible for getting motile organisms where we find them. In 
order to understand the mechanism of distribution of these forms, 



216 Light 

it is necessary to analyze the reactions involved. Many practical 
applications of such knowledge suggest themselves, such as the control 
of insect pests. 

In attempting to interpret the locomotory reactions of organisms 
and to apply them to natural situations, the following considerations 
should be kept in mind: (1) Responses to individual stimuli are 
usually limited by the influence of other conditions. Thus the posi- 
tively phototactic moth does not attempt to fly to the sun. (2) The 
observed distribution may be the result of movement without direct 
orientation. The reduced locomotory activity of wood lice in situa- 
tions where moisture is high results in a tendency for these animals 
to congregate in moist places although they are not specifically 
oriented to them, just as the traffic on a main highway becomes 
denser in a bad stretch of road because the speed of the cars is 
reduced. (3) For an oriented response the orienting factor must 
provide a stimulus above the threshold of sensitivity, and either the 
direction of the flux of the orienting force or the direction of a gradient 
produced by it must be perceptible to the organism. The tempera- 
ture gradient in the ocean, for example, may be shown to be below 
the threshold for the response of zooplankton (Clarke, 1934). (4) 
Factors whose gradients are below the threshold often exert an in- 
direct effect by controlling the sign or the speed of the response to 
another factor. (5) Responses may similarly be altered under 
changed internal physiological conditions, as before or after feeding, 
or breeding. Within the same species the males, females, and various 
immature stages sometimes orient quite diversely. Thus, in gen- 
eral, we see that the distribution of plankton in the water, of microbes 
in the soil, or of insects in the vegetation is the result of a complex 
interplay of the orienting factors of the environment acting directly 
and indirectly on the changing physiological state of members of the 
population. 

Periodicity 

Diurnal Periodicity. A good number of the fundamental rhythms 
in nature are related to the light factor. Many animals and plants 
exhibit a 24-hour cycle in their activities; this has long been known 
as diurnal periodicity. However, the term diel periodicity may be 
substituted if confusion arises from the fact that diurnal is also used 
for daytime activity as opposed to nocturnal for nightime activity. 
The most fundamental diurnal rhythm is that of photosynthesis itself, 
which necessarily fluctuates because of the daily change of light. 
Many plants exhibit other more specialized reactions to the alterna- 



Diurnal Periodicity 217 

tion of day and night, such as the opening and closing of flowers and 
the folding of leaves. 

Although often not conspicuous, a very large number of the lower 
animals are controlled in their activities by the change in light from 
day to night. This is often a matter of simple photokinesis. The 
animals are more active in the light, less active in darkness or vice 
versa, but behavior may also be modified by concomitant diurnal 
changes in other factors such as temperature and humidity. Among 
the higher animals the reactions are often more elaborate, and some- 
times the daily rhythm of activity is only indirectly related to light. 
Many of the relatively defenseless forms, such as mice, come out to 
forage chiefly at night when they are less likely to be detected by their 
enemies. As a further complication many of the predaceous animals 
are nocturnal as an adaptation to the night activity of their prey or 
to the fact that they can stalk their prey more successfully under 
cover of darkness. A patch of woods may be inhabited by two sets 
of animals which practically never meet because one set is active 
only during the day and the other active only during the night (Park, 
1940). 

The fact that a diurnal rhythm in the activity of an organism is 
sometimes deeply ingrained can often be demonstrated by bringing 
the organism into the laboratory and observing it under constant or 
under changed conditions of illumination. In one such experiment 
the deer mouse, Pcromtjscus, which is normally active at night and 
quiescent during the day, continued to display a diurnal rhythm in 
its behavior after seven months in continuous darkness. This and 
other experiments summarized by Welsh (1938) and by Park (1941) 
indicate that certain internal physiological processes have become 
attuned to a 24-hour cycle. In many organisms this rhythmicity per- 
sists for long periods and tides over periods when the usual controlling 
environmental stimulus fails to occur. Under normal conditions the 
timing of the periodicity is re-enforced by changes in light or other 
factors each day. 

The diurnal shift in the activity of animals frequently results in 
significant changes in their position in the community. Perhaps no 
better illustration of this phenomenon could be found than the vertical 
migration of zooplankton in the sea and in lakes. In general, vast 
numbers of copepods and other planktonic animals tend to swim to- 
ward the surface at night and to move downward to deeper levels 
during the day. The diurnal changes in vertical distribution of fe- 
males of the copepod, Calanw, in the Clyde Sea area are indicated 
in Fig. 6.17. The time of ascent in the evening and of descent in the 



218 



Light 



morning, as well as the depth to which the animals migrate, is corre- 
lated with the differences in time of sunset and sunrise and in the in- 
tensity of the noon sun during January and July. The complexity of 
the response to light is revealed by the fact that in this same area, the 
males and the various young stages of the same species exhibit quite 
different patterns of behavior. 



4ipm 7 pm 10 pm 



25-26th January 

1 am 4 am 7 am i 10 am 1 pm 4 pm 




!M2thJuly 



50 20 20 50 



Fie. 6.17. Diurnal vertical migration of female Calanus finmarchicus in the Clyde 
Sea area during 24-hour periods in January and in July. The abundance of the 
population is indicated by the width of the figures. The times of sunset and 
sunrise are indicated by arrows. (Nicholls, 1933, Copyright, Council of Marine 
Biol. Assoc. of United Kingdom.) 

Reactions to gravity and to temperature change often modify the 
course or the extent of the vertical migration (Clarke, 1933). The 
various theories dealing with the reactions of the zooplankton to the 
factors believed to control vertical migration have been summarized 
by Gushing ( 1951 ) , The magnitude of the migration is indicated by 
the fact that the copepod population may often move more than 100 
m in a vertical direction. This represents a journey of more than 
15,000 times the animal's own length twice each day. Even more ex- 
tensive diurnal vertical migrations are carried out by euphausiids, 
fish, and perhaps other large active forms. The widespread occur- 
rence of changes in level of huge populations in the open ocean, dur- 
ing the course of the day, has been revealed by midwater echoes re- 
ceived by the underwater acoustical equipment of ships. A discus- 
sion of the kinds of animals probably responsible for this "deep scat- 
tering layer" and its vertical oscillation is presented by Moore ( 1950). 

Similar vertical movements occur in terrestrial environments al- 



Lunar Periodicity 219 

though these are generally less extensive. Many animals move from 
the surface to deeper levels in the soil at regular periods each day. 
Others emerge from under the ground litter at definite times during 
the diurnal cycle and ascend the vegetation. Harvestmen, or "daddy- 
long-legs" (Leiobunlum rotundum), in an English oak wood were 
observed to descend from the tree trunks in the evening to the forest 
floor where they hunt their prey and to move up onto the trees again 
in the early morning (Todd, 1949). Many other insects move reg- 
ularly from lower levels in the herbs and shrubs to higher positions 
in the trees at dawn and return at dusk; others migrate in the reverse 
manner. These changes in levels of whole populations, both in the 
water and on land, have profound repercussions on prey-predator 
relations and other interdependencies among the inhabitants. For 
a further discussion of these aspects of stratification and periodicity 
in the community, the reader should refer to Allee et al. (1949, Ch. 
28). 

Lunar Periodicity. Since the days of classical Greece interest has 
been attracted to the correlation of certain animal activities with pe- 
riods of the moon. Oppian in the time of Aristotle wrote: 

The shellfish which creep in the sea are reported, all of them when the 
moon waxes, to fill up their flesh proportionately to her disk, occupying then 
a bigger space, On the other hand when she wanes they shrivel and their 
members grow thinner. 

It is true that in the Red Sea the gonads of the sea urchins ( the edible 
portion) do enlarge during the period of the full moon, but the be- 
lief was spread fallaciously around the Mediterranean and elsewhere 
that the lunar cycle controlled many more animal activities than is 
actually the case. One even meets the statement occasionally that 
the 28-day menstrual period in man harks back to our marine an- 
cestors. Since we know that the menstrual cycle in other mammals 
has very different periods, the agreement of the human period with 
the 28-day lunar cycle is merely a coincidence. 

Modern investigation has shown that the activity of certain widely 
different types of organisms, usually in relation to the reproductive 
cycle, shows a correlation with the moon. The striking nature of 
these lunar periodicities is well illustrated by the fluctuation in the 
abundance of conjugants produced by a ciliate living as an ectopara- 
site on the gills of a fresh-water mussel (Fig. 6.18). The distinct 
peaks occurred regularly on the days following the new moon and 
were not correlated with temperature or other known environmental 
changes. 



220 



Light 



100 

90 
80 

03 

70 



50 
40 
30 - 
20 - 
10 - 



19 



20 



19 



20 



17 





18 



17 





15 



14 





22 



15 

i 



Sept. Oct. Nov. Dec. Jan. Feb. 

FIG. 6.18. Average daily number of conjugants of the ciliate Conchoplrthirius 
lamellidens on the gills of the fresh-water mussel Lamellidens marginalis. Dates 
of new moon (top) and of peak numbers are shown. (Ray and Chakraverty, 

1934, Nature.) 

Most of the organisms exhibiting lunar periodicities are marine and 
hence may be affected by the amplitude of the tide which is greatest 
at times of new moon and full moon (spring tides) and smallest at 
the times of the quarter moon (neap tides). For organisms living 
within the influence of the tides it is obviously difficult to determine 
how much of the effect may be due to moonlight itself and how much 
to the action of the tidal cycle. The marine alga, Dictyota, for ex- 
ample, produces its gametes at the time of the full-moon spring tide. 
The spawning of a number of marine polychaete worms shows various 
time correlations. The palolo worm inhabiting the waters of the 
South Pacific islands comes to the surface in great numbers on the 
last quarter of the moon during October and November, producing a 
luminescence and discharging eggs and sperm into the water. The 
natives know of the occurrence of this swarming of the palolo worm 
and take advantage of the opportunity to scoop up large quantities 
of these animals for food. The worms swim about in small circles in 
dense masses, giving the sea an appearance of spaghetti soup. The 
natives gather the worms in crude baskets and celebrate the occasion 
with religious rites and feasts. 

The Bermuda fireworm puts on a similar display of fireworks in the 
shallow water early in the evening at the time of full moon. The 
reaction appears to be set off by the drop in light intensity following 
sunset (Huntsman, 1948). Professor E. L. Mark, who was for years 



Lunar Periodicity 221 

director of the Bermuda Biological Station for Research, took a special 
interest in studying the swarming of this marine polychaete. He 
found that swarming and luminescence began at about 55 minutes 
after sunset and lasted for half an hour. On one occasion when Pro- 
fessor Mark was traveling to Bermuda he realized that he would ar- 
rive on a day when the fireworms were due to appear. He interested 
his fellow passengers in the phenomenon, telling them that the timing 
was so exact that one could set one's watch by it and invited a large 
group of people to come to the shore that evening. When the moment 
arrived for the beginning of the display, not a worm was in sight, 
nor was there after 5 minutes, 10 minutes, and more. Soon people, 
feeling that they had been hoodwinked, began moving away, but 20 
minutes after the appointed time the worms appeared in great num- 
bers and a spectacular demonstration was given for the visitors who 
still remained. Not until the next day was Dr. Mark's embarrassment 
relieved when it was discovered that during the winter Bermuda 
time had been changed from local sun time to zonal time, resulting 
in setting the Bermuda clocks ahead 19 minutes. 

Perhaps the most fascinating of all the responses of animals to the 
lunar or tidal cycle is that exhibited by the grunion or California 
smelt. On the three or four days after the spring tides from April to 
June the grunion swims in on the beaches of southern California to 
lay its eggs in the sand. During this period the higher of the daily 
high tides comes at night. About an hour after the water reaches its 
highest level on each of these nights the fish allow themselves to be 
carried up on the beach by the waves. As each wave recedes, an 
observer can discern hundreds or thousands of fish left behind, wrig- 
gling in the wet sand (Fig. 6.19). Within a few seconds the females 
have burrowed tail first into the sand where they deposit the eggs 
while the males curl around them discharging the sperm. When the 
next wave comes in, the fish are washed out to sea once more. Since 
the eggs have been discharged during the period of descending tides 
after the highest spring tides and during the hours following high 
tide each night, the waves do not reach them again during the next 
two weeks. The eggs are thus left undisturbed to develop in the 
warm, moist sand. When the next spring tide occurs, the eroding 
surf of the rising tide uncovers the eggs, now ready to hatch out, and 
the young larvae swim away. In order for this complicated relation- 
ship between the life cycle of the fish and the tides to be carried out, 
the fish, which ordinarily remain in the waters offshore, must be 
stimulated in some way to move in to the beach on the proper day 
and to allow themselves to be stranded on the shore by the waves 



222 Light 




FIG. 6.19. Flashlight photograph of grunion (Leuresthes tenuis) spawning on 

Malibu Beach near Los Angeles, California. The females bury themselves tail 

first in the sand. The males then circle around them and fertilize the eggs. 

(Photo by Moody Institute of Science.) 

only during the proper hour, Just how this timing is controlled has 
not yet been discovered despite a considerable amount of study. The 
complexity of the interplay of the tidal and lunar influences and the 
remarkable precision of the responses of the grunion are indicated in 
Fig. 6.20. 

The foregoing are representative of the varied occurrences of lunar 
periodicities in nature of which many more instances could be cited 
(Korringa, 1947). A definite control of the timing of the reproduc- 
tive cycle is usually indicated, and frequently other special activities 
such as swarming or luminescent discharge are involved. The mech- 
anism by which the timing is controlled is far from clear, however. 
For some organisms the changing amount of moonlight may be of 
chief importance either directly or as a degree of contrast with sun- 
light or starlight. Since the intensity of full moonlight is more than 
1(H of noon sunlight, it is well above the threshold for the response 
of many organisms. As an alternative the cyclic change in the rela- 
tion of the moon's gravity has been suggested as possibly controlling 
the activity of fish and other organisms. Many sportsmen strongly 
believe that such lunar gravitational fluctuations influence the success 
of their fishing. At the present time, however, we have no evidence 



Lunar Periodicity 



223 




224 Light 

that the sense organs of any type of animal could respond to the dif- 
ferences in the moon's gravitational effect. 

For marine animals of the littoral zone the moon's influence often 
appears to be mediated at least in part through the change in the 
tides, but certain lunar periodicities occur in regions where tides are 
slight or lacking. One great difficulty in studying this problem is 
that often the behavior pattern persists for a while under changed 
conditions, as when the moon is obscured, apparently because of a 
correlation with an internal rhythm. Much more investigation will 
be required before these intriguing periodicities are completely un- 
derstood. 

Seasonal Periodicity. The seasonal activities of plants and animals 
are sometimes due to changes in temperature and are sometimes con- 
trolled by the cycle of dry and rainy seasons; seasonal periodicity may 
also be influenced by the light factor. The seasonal effect of light 
is not so much due to differences in intensity as it is to differences in 
total amount of light and in the relative lengths of day and night. 
The response of organisms to daylength is known as photoperiodism. 
The most striking manifestation of photoperiodism is the control of 
the reproductive cycle in certain plants and animals, although other 
life processes are involved in some instances. In these organisms the 
reproductive phase of the life cycle is initiated by days that are shorter 
or longer than certain critical lengths. Under natural conditions short 
days are accompanied by long nights and vice versa. Experimental 
manipulation of the photoperiod has revealed facts indicating that the 
length of the night may be more influential than the length of the day 
in controlling the photoperiodic responses of at least some of the 
plants and animals, but the precise mode of action of the light factor 
has not yet been determined (Farner et al., 1953). 

Photoperiodism was first discovered in relation to plants. Species 
that flower only when the days are longer than a certain number of 
hours and the nights are correspondingly short are known as long- 
day plants. Short-day plants, on the other hand, flower naturally only 
under conditions of short days and long nights. However, the re- 
productive cycle of certain other species is not affected by daylength; 
these are referred to as indeterminate or indifferent plants. This 
aspect of periodicity in the light factor controls the season at which 
long-day and short-day plants flower in any locality, and it also in- 
fluences the geographical distribution of the species. 

Long-day plants flower naturally only in the middle or high lati- 
tudes during the late spring or early summer. Familiar examples 
are the radish, iris, red clover, evening primrose, spinach, the smaller 



Seasonal Periodicity 225 

cereals, and timothy. If such plants are grown under daylengths 
shorter than the critical photoperiod, stems tend to be shorter and 
flowering is suppressed (Fig. 6.21). Other effects may also be pro- 
duced in the plant, as illustrated by long-day potatoes which produce 
the best tubers when daylength is below the optimum for shoot 
growth. 




FIG. 6.21. Control of flowering and of vegetative growth by daylength in 

timothy, a typical long-day plant. The daily exposures (in hours) to light are 

indicated on each container; C natural daylength. Photo taken in July. 

(Evans and Allard, 1934.) 

In the growing season outside the tropical zone short days occur 
both in the early spring and in the late summer. Certain short-day 
plants require a long growing period before they are sufficiently 
mature to react to the flowering stimulus, and such plants can flower 
only in the shorter days following the summer solstice. Familiar 
plants that can bloom naturally only late in the year are tobacco, 
goldenrod, aster, dahlia, ragweed, cosmos, and chrysanthemum. In 
daylengths above the critical abnormally great vegetative growth 
takes place and flowering is much delayed or entirely inhibited (Fig. 
6.22). Short-day onions and beets develop the largest storage organs 
under photoperiods that are longer than those best for the growth of 
the upper part of the plant. 

The plants that bloom in the short days of early spring are mostly 
perennials in which the flower buds were set the previous autumn, 
The few annuals blooming at this season either germinated during 
the winter or are so small that they need little time for vegetative 



226 Light 




FIG. 6,22. Control of flowering and of type of vegetative growth by daylength 
in garden chrysanthemum, a typical short-day plant. The daily exposures (in 
hours ) to light are indicated on each container; C = natural daylength. Photo 
taken in July. Flowering occurred as follows: 10 hr, July 6; 12 hr, Oct. 14; 13 
hr, Oct. 19; 14 hr, Oct. 31; natural day, Oct. 31. (Allard and Garner, 1940.) 



growth before flowering. Early spring flowers growing on the floor 
of the deciduous forest are able to take advantage of the great amount 
of light available before the tree leaves are fully developed. 

Since short days occur later in the summer at higher latitudes, the 
northward distribution of short-day plants may be stopped by the 
lack of sufficient time for the ripening and hardening of the seed be- 
tween the occurrene of the critical daylength and the occurrence of 
a killing frost that terminates the growing season. Ragweed, for 
example, which flowers when the days have shortened to 14% hours, 
will begin its reproductive phase about the first of July in Virginia, 
and hay-fever sufferers notice its pollen in the air by the middle of 
August. In northern Vermont the summer days are not reduced to 
14% hours until after the first of August, with the result that seed 
formation is not completed before cold weather, and consequently 
ragweed is unable to establish itself in abundance at this latitude. 
The distribution of long-day plants toward the equator is limited by 
the reverse action of this aspect of the light factor. Species of Sedum 
that require a daylength of 16 hours or more will flower abundantly 
in Vermont but will not bloom in Virginia (Naylor, 1952). 

Plant species in which flowering is not controlled by daylength 
also occur at all latitudes, but all plants growing at high latitudes must 
be able to tolerate long days and short nights since these light condi- 
tions exist during the growing season, and tropical plants must be 



Seasonal Periodicity 227 

able to complete their development under photoperiods of about 12 
hours* duration. Since the vegetation of intermediate latitudes is 
composed of a mixture of long-day, short-day, and indifferent plants, 
the seasons of vegetative growth, flowering, and seed dissemination 
vary among the species of the community, and competition is reduced 
accordingly. In some instances genetic strains of the same species 
growing under different climatic conditions respond differently to 
given photoperiods, apparently, as an adaptation to differences in the 
time of onset of adverse conditions. The possibility of intraspecific 
variation of this sort among both plants and animals in their response 
to all types of environmental factors must constantly be kept in mind. 

The control of the reproductive cycle by the length of day is a re- 
sult of a complicated balance between life processes that go on in the 
light and in the dark (Leopold, 1951). Since the intensity of light 
which influences the photoperiodic response is far below that needed 
for photosynthesis, we know that the 'reactions responsible are quite 
different. The delicate nature of the processes involved are indicated 
by the fact that only a few minutes, or even a few seconds, of light 
during the middle of a long night may reverse the flowering reaction 
of the plant. The tasseling of sugar cane can be inhibited by a short 
light period during the night, and the possibility of retarding the 
ripening of the crop until the most favorable time for harvesting by 
sweeping the cane fields with search lights has been investigated by 
Hawaiian planters. In greenhouse plants and other commercial 
species the timing of the production of flowers, seeds, fruits, or storage 
organs is controlled artificially by means of the photoperiodic re- 
sponse. 

Photoperiodism similarly plays an important role in the life cycles 
of many kinds of animals and, interestingly enough, also most fre- 
quently controls the reproductive cycle. However, wing production 
in aphids, metamorphosis in mosquitos, pelage changes in fur bearers 
(Lyman, 1942), and other life processes are known to be influenced 
by daylength. The most extensive studies have been carried out on 
mammals and birds, and these have been reviewed by Bissonette 
(1936) and Burger (1949). Trout that ordinarily spawn in Decem- 
ber can be induced to lay their eggs in August by artificially chang- 
ing the daylength. The fresh-water pulmonate snail, Lymnaea 
palustris, will not lay eggs on an 11-hour day but lays abundantly 
when the days are 13% hours long or longer. The control of repro- 
duction by daylength in this species rather than by temperature was 
neatly demonstrated under natural conditions by the fact that snails 
living in a spring with practically no seasonal change in temperature 



228 Light 

began laying eggs during the same week as did snails inhabiting a 
shallow ditch that underwent rapid vernal warming (Jenner, 1951). 
Very little additional investigation of photoperiodism among the in- 
vertebrates has been made, but this reaction to light may eventually 
be found to play an important role in controlling the life cycle of 
many of the less conspicuous animals. Among mammals certain 
forms, such as ferrets, are brought into breeding condition by the 
long days of spring, but others, such as deer, come into heat during 
the short days of autumn. 

Photoperiodism in birds was first discovered as a result of studies 
of migration. It had long been realized that the arrival of birds in 
the spring was not closely related to temperature or other common 
aspects of the weather since birds are often caught by late winter con- 
ditions. A similar difficulty arose in explaining the start of the south- 
ward migration. Many birds leave the northern regions during July 
or August when the temperature is high, and when an ample food 
supply is still available (Fig, 6.23). The length of clay was finally 




Photo by Allan D. Crmckshank, National Audubon Society 

FIG. 6.23. Tree swallows flocking previous to migration. 



Seasonal Periodicity 229 

recognized as one environmental factor that changed regularly year 
in and year out and that could account for the exact timing of the 
migration regardless of the weather conditions. Present evidence 
indicates that the effect of the daylength is brought about through 
controlling the amount of light reaching a sensitive tissue in the 
bird's body (the pituitary gland). Under natural conditions the 
larger amount of light received during the long days of spring causes 
an increase in the size of the gonads and also, in some way as yet un- 
known, sets off the northward migration. Following the breeding 
season the diminishing daylengths of late summer and autumn cause 
a decrease in gonad size and provide the stimulus for the southward 
migration. Evidence on the mode of action of light and other factors 
on bird migration has been summarized by Farner (1950). 

Although photoperiodism thus appears to be basically involved in 
the annual stimulus for the migration of birds, it does not explain the 
evolutionary origin of the migratory habit nor, most mysterious of 
all, how the birds find their way. Once established, however, migra- 
tion would tend to be perpetuated by its advantages. At higher lati- 
tudes the longer days make more time available for nest building 
and for feeding the young. The young grow more rapidly, and hence 
the whole process of rearing the family, with its attendant dangers, 
is completed more quickly than would be the case nearer the equator. 
By leaving the north during the winter, on the other hand, the birds 
avoid the hazards of low temperatures and shortage of food. 

The migration of birds is evidently a secondary effect of the in- 
fluence of light. The primary effect is in the control of the breeding 
cycle since this can be demonstrated in species of birds that do not 
migrate. The occurrence of breeding in the English sparrow at dif- 
ferent latitudes over the face of the earth is indicated in Fig. 6.24. 
Near the equator some breeding takes place in every month of the 
year. In areas farther north and farther south reproduction tends to 
be confined to the months of the year with longer days. North of 
50 N latitude breeding is strongly centered in the month of May with 
no breeding at all during the months of short daylength, and south 
of 50 S latitude most breeding takes place in December. Outside of 
the tropics, therefore, the light factor plays an important role in con- 
fining the breeding period to the months of the year when reproduc- 
tion is most successfully carried forward. Since short daylength pre- 
vents birds from entering the breeding condition too early and the 
long days of midspring act to speed up the laggards, the whole popu- 
lation tends to breed within the same favorable period ( Bartholomew, 
1949). 



230 



Light 

1 . " ." ! , IV , v , v ' v'lyni ix x XT xii i ii in iv v vi 



T 1 1 T 



1 \ 1 1 




10 20 N 

0- 10 Ni 

0-10S. 

10 -20Si 

20 -30 Si 

30 - 40 $ 

40 - 50 S 

50 60 S 




J L 



i ii in iv v vi vii Vm 1 ix ' x ' xi 'xn 1 i ' n ' m' iv 1 v ' vi ' 

FIG. 6,24. Relative intensity of breeding of the English sparrow in each month of 
the year at the indicated latitudes. (Baker, 1938.) 



Ultraviolet Light 

The small fraction of the sun's radiation that reaches the earth as 
ultraviolet light has certain very special biological effects, and some 
of these may be of ecological significance. The sensitivity of the bee's 
eye to this part of the spectrum and its use in orientation has already 
been mentioned. The population size of certain land animals in north 
central United States was found by Shelford (1951 and 1951a) to be 
correlated with the intensity of ultraviolet light, although the reactions 
that underlie the correlation are not known. The bactericidal 
action of ultraviolet is familar and causes the destruction of micro- 
organisms that are excessively exposed to the direct rays of the sun. 
Ultraviolet radiation produces sunburn, or erythema, in man and per- 
haps causes harmful effects in other animals under natural condi- 
tions, although this question has not been investigated from the 
ecological viewpoint. Ultraviolet also brings about the production of 
vitamin D with its antirachitic effect. Vitamin D is formed by the 
irradiation by ultraviolet light of certain sterols, or fatty substances, 



Ultraviolet Light 231 

that occur in both plants and animals. The relative effect of different 
wavelengths in producing the foregoing influences is shown in Fig. 
6.25. Although the curves differ very much from one another 
throughout most of the range, it is clear that in no instance is ultra- 
violet radiation effective at wavelengths longer than about 3100 A. 




250 260 



300 



310 



320 



270 280 290 
Wavelength 

FIG. 6.25. Relation between wavelength in the ultraviolet region and the bac- 
tericidal, erythemic, and antirachitic effects of ultraviolet radiation, together with 
the absorption of ergosterol. ( Coblentz, 1939, Copyright, J. Am. Medical Assoc. ) 

Under ordinary conditions no measurable radiation from the sun 
reaches the earth's surface at wavelengths shorter than 2950 A. It is 
clear then that under natural conditions the whole action of ultraviolet 
must occur between 3100 A and 2950 A. Under the best circum- 
stances solar radiation is extremely weak in this region, and when dust 
or smoke is abundant the shorter wavelengths are still further re- 



232 Light 

duced. During the winter in the neighborhood of cities no measur- 
able radiation shorter than 3100 A can be found, and hence no ap- 
preciable action of ultraviolet can take place at that season. When 
birds that ordinarily migrate south were retained in Alberta, they 
died during the winter apparently from lack of vitamin D. Neverthe- 
less, we have little evidence that the lack of ultraviolet in winter is 
generally of crucial ecological importance, but the matter should be 
investigated further. 

In the aquatic environment ultraviolet is still further reduced by 
the very rapid rate at which it is absorbed even in the clearest water. 
Although in some quarters popular imagination has fixed upon the 
idea that ultraviolet is the most penetrating region of the spectrum, 
actual measurements have shown that radiation at 3100 A is reduced 
to 10 per cent of its surface value at a depth of 15 m in the extremely 
clear water of the Mediterranean and at 1 m in Gullman Fjord on the 
coast of Sweden. The shorter and biologically more active wave- 
lengths are reduced even more rapidly by absorption and scattering 
in the water (Jerlov, 1951). In most natural waters the effectiveness 
of ultraviolet diminishes very rapidly with depth. Carefully con- 
trolled experiments have revealed no bactericidal action below 1 m 
and probably none at depths greater than 20 cm even in clear ocean 
water. The ecological effect of the lethal action of ultraviolet in very 
shallow water awaits further study. It is possible that the bacterial 
population on the bottom mud in the littoral zone is seriously affected 
when exposed at low water. 

How can we account for the formation of vitamin D in fish liver 
oils? Cod and halibut whose oils are particularly rich in vitamin D 
live far below the depth at which any measurable ultraviolet could 
penetrate. It has been suggested that the vitamin D might reach 
the fish livers through a food chain starting with the phytoplankton 
and the zooplankton that live near the surface of the sea. Since cod 
and halibut are bottom feeders, a long food chain must be postulated 
to extend to the forms on which these fish live. Another suggestion 
is that the vitamin D formed in the Sargassum weed as it floats near 
the surface of the tropical oceans finds it way via the food chain into 
such species as the cod. For the most part we know that the animals 
associated with the Sargassum weed do not eat the weed directly but 
live on plankton or materials accumulating on the surfaces of the 
fronds. In addition we know that a sharp temperature barrier exists 
between the regions where the Sargassum weed is abundant and 
areas inhabited by such fish as the cod and halibut. The amount of 
vitamin D in the liver oils of these species is very great. It seems in- 



Land Plants 233 

conceivable that the supply could have resulted from such tenuous 
food chains as are described above, even if the fish were able to retain 
the vitamins in their bodies as mammals are not. Nor has any method 
been shown by which an animal could manufacture vitamin D with- 
out the presence of ultraviolet light. The actual source of the large 
quantities of vitamin D in the fish liver oils thus remains a mystery. 
It emphasizes again how little we know about ecological influences 
of this region of the spectrum, both in the water and on land. 

Ecological Aspects of Photosynthesis 

Light is fundamentally important as the essential direct source of 
energy for the growth of all green plants and the purple bacteria. 
The photosynthetic plants form the first step in the ecological cycle 
in every natural situation. They are the first link in the food chain 
and hence are the base of the production pyramid that will be dis- 
cussed in greater detail later on. 

Land Plants. On the surface of the land enough light is received 
everywhere for the growth of plants of some sort. Within the soil 
and in the interior of caves light is insufficient to allow photosynthesis. 
A striking illustration of the limiting action of the light factor is found 
in caves to the walls of which electric lamps have been secured for 
the benefit of tourists. Within the cone of light immediately around 
each lamp mosses have grown from spores brought in by air currents. 
These "islands" of plant growth present a sharp contrast to the com- 
plete lack of vegetation in the remainder of the cave. 

Although illumination is generally sufficient for photosynthesis over 
the entire surface of the globe, the local distribution of individual 
species of plants is definitely influenced by differences in the avail- 
ability of light. Species that can grow in shady places are termed 
tolerant, and, although the degree of tolerance is affected by soil 
moisture, temperature, and other factors, the illumination is the prin- 
cipal controlling influence. Plants that require strong illumination 
and will not survive or develop in reduced light are referred to as 
intolerant species. Trees such as the spruce, hemlock, beech, and 
sugar maple, shrubs, such as spicebush, and herbs, such as blood- 
root, can grow in deep shade and are examples of tolerant species. 
The sugar maple can photosynthesize adequately in situations where 
the illumination is reduced to less than 2 per cent of full sunlight. 
Birch trees, poplars, willows, and several species of pine, as well as 
many shrubs and herbs such as sumac, bluestem grasses, and sun- 



234 



Light 



flowers are intolerant of shade. The ponderosa pine, for example, 
requires a light intensity equal to at least 25 per cent of full sunlight 

Intolerant species cannot develop in the shade of a dense stand 
of their own or other species. Ecological consequences of this fact 
for the plant community will be discussed further in later chapters. 
Even among somewhat tolerant species the first plants to become es- 
tablished in an open area will grow to better advantage than indi- 
viduals subsequently developing around them because of the com- 
petition for light and other needs. Differential growth often results 
in "pyramiding" in the development of such a group of plants (Fig. 
6.26). As we have seen in an earlier section, the light on the floor 




FIG. 6.26. Pyramiding due to competition exhibited by a group of spruce trees 
growing in a Michigan bog. 

of the temperate deciduous forest becomes seriously reduced when 
the trees leaf out. Many of the smaller plants are adapted to a period 
of rapid growth in the early spring while light on the forest floor is 
sufficiently intense. In the marginal rainforest of Panama a dense 
undergrowth is possible because during the dry season the leaves fall 
from the trees and allow light to penetrate. In the equatorial rain- 
forest of Columbia, on the other hand, trees shed their leaves in- 



Aquatic Plants 235 

dividually and a continuous canopy is maintained, with the result 
that little vegetation can grow on the forest floor. 

In such rainforests many smaller plants have gained access to suf- 
ficient illumination by the evolution of the epiphytic habit, that is, 
by growing in the crotches or on the horizontal branches of the trees. 
The vertical distribution of the many species of bromeliads in the 
forest trees of Trinidad reveals their varying degrees of tolerance 
(Fig. 6.27). One group of species grows only near the tree tops in 
situations fully exposed to the sun, a second group is found at inter- 
mediate levels in partial sunlight, and a shade-tolerant third group 
inhabits the lower branches of the forest where the illumination is 
quite inadequate for the other groups. 

Aquatic Plants. Reduction of light presents even more serious 
problems in the aquatic environment. As we have seen illumination 
diminishes rapidly with depth even in clear water and becomes 
changed in spectral composition and in other respects. Plants at- 
tached to the bottom in the marine environment consist principally of 
algae with a few species of vascular plants such as the eel grass. In 
fresh water vascular plants as well as algae are well represented in 
the submerged vegetation. 

Early investigators reported an apparent color zonation in the depth 
of occurrence of attached plants. In Puget Sound, for example, the 
green algae are generally the most abundant in shallow water, brown 
algae dominate the zone from 5 to 20 m, and red algae are usually 
most numerous in depths of 10 to 30 m. These color types of algae 
often occur at depths where the complementary color of the penetrat- 
ing daylight predominates red algae, for example, tend to be abun- 
dant in deep water where the blue or green component of daylight is 
the strongest. It was formerly believed that the predominating color 
of the light controlled the depth distribution of the color types of 
algae because the plants would absorb light of complementary color 
more efficiently. This generality was referred to as chromatic adap- 
tation. Many exceptions occur, however, and we now know that the 
wavelength of light does not control in any precise way the depth of 
distribution of algae on the basis of their color ( Dutton and Juday, 
1944). Furthermore, the pigment of algae living in weak light deep 
in the water is sufficiently thick to absorb all the light incident upon it, 
regardless of the part of the spectrum in which it occurs. Evidence 
exists that some of the light energy adsorbed by pigments other than 
chlorophyll can be transferred to the chlorophyll present and thus 
enhance the ability of certain algae to live in weak blue-green light 
(Smith, 1951, Ch. 13). 



236 



Light 




Group I (exposure) 




Group II (sun) 




Group III (shade tolerant) 

FIG. 6.27. Vertical distribution of various species of bromeliads in the forest trees 
of Trinidad. ( Pittendrigh, 1948.) 



Aquatic Plants 237 

When algae are grown in the laboratory in light of different colors, 
they often tend to change color, and this response has also been 
referred to as chromatic adaptation. Experiments have shown that 
individual plants photosynthesize most efficiently and grow best in 
that color and in that intensity in which they have been living. In 
the course of time fixed algae can adapt themselves to habitats with 
illumination of widely different conditions of color and intensity. In 
view of the great range in the transparency of natural waters, it is not 
surprising that the maximum depth at which attached plants can live 
varies widely in different situations (Table 15). In general benthic 

TABLE 15 
MAXIMUM DEPTH IN METERS FOR GROWTH OF ATTACHED PLANTS 

Crater Lake, Ore. 120 

Crystal Lake, Wis. 20 

Trout Lake, Wis. 12 

Mediterranean Sea 160 

Challenger Bank 100 

Off Iceland 50 

Puget Sound 30 

Baltic Sea 20 

Off Cape Cod 10 

TABLE 16 

COMPENSATION DEPTHS IN METERS FOR PHYTOPLANKTON 
(for midday periods) 

Sargasso Sea 100 

English Channel 35 

Gulf of Maine 30 

Off British Columbia 19 

Trout Lake, Wis. 16 

Woods Hole harbor 7 

plants will not grow at depths at which the light intensity is less than 
0.3 per cent of the surface value. 

A glance at a chart of the oceans or of a typical lake reveals that 
only in a very narrow fringe around the margin is the water suffi- 
ciently shallow for enough light to reach the bottom for the growth of 
plants. We are forced to the realization that in any deep body of 
water far more organic matter is synthesized by the phytoplankton 
present everywhere in the surface layers than by the benthic plants 



238 Light 

limited to the littoral zone. Photosynthesis, the first step in the 
ecological cycle of the sea upon which all marine life ultimately 
depends, is carried forward primarily by the minute diatoms and 
other microscopic planktonic plants. The growth of the phyto- 
plankton is limited not only by the rapid diminution of light with 
depth but also by the fact that vertical movements of the water sub- 
ject the plants to continually changing light conditions. 

The greatest depth at which phytoplankton can grow successfully 
in any body of water may be ascertained by suspending bottles 
containing samples of the plant population at various levels beneath 
the surface. Photosynthesis going on within the bottle adds oxygen 
to the water, and respiration taking place simultaneously removes 
oxygen from the water/ The rate of respiration alone may be meas- 
ured by placing at each level a blackened bottle from which all light 
is excluded. The sum of the gain in oxygen in the clear bottles and 
the loss in the dark bottles gives a measure of total photosynthesis. 
The increment of oxygen in the clear bottles is the amount produced 
by photosynthesis minus the amount consumed in respiration and 
represents the actual increase in energy content or growth of the plants. 

With the diminution of light in any environment photosynthesis 
is reduced but respiration remains approximately the same, pro- 
vided, of course, that temperature and other factors are essentially 
unchanged. For each species a point is reached in the reduction 
of the illumination at which the rate of photosynthesis is just equal 
to the rate of respiration; this is known as the compensation point or 
better as the compensation intensittj. At light intensities below this 
value photosynthesis may still go on but the plant is fighting a losing 
battle: it cannot survive indefinitely under these conditions because 
the loss of energy duetto the catabolic processes represented by 
respiration exceed the gain in energy brought about by the anabolic 
process of photosynthesis. With further light reduction a minimum 
intensity is reached below which no photosynthesis at all can take 
place. 

In the aquatic environment the level in the water at which the 
compensation intensity is found is called the compensation depth. 
Sample values for phytoplankton based on measurements in the 
middle of the day are presented in Table 16. For phytoplankton in 
general the compensation intensity has been found to be about 1 per 
cent of the value of full sunlight at the surface. In Fig. 6.28 the 
change in the amount of photosynthesis at various depths during the 
course of the day is plotted for a type of diatom representative of the 
marine plankton. It will be observed that during the noon hours the 



Aquatic Plants 239 

illumination close to the surface was excessive and depressed photo- 
synthesis. The highest rate of oxygen production occurred at 5 m 
in the middle of the day. No significant amount of photosynthesis 
occurred at any depth before 0600 in the morning or after 2200 and 
none below 35 m at 1400. 




o 



10 



noon 14 

Time > 

FIG. 6.28. Photosynthesis of a species of phytoplankton (Coscinodiscus) at the 

indicated depths during the course of the day off Stoke Point, England. (Jenkin, 

1937, Copyright, Council of Marine Biological Assoc. of United Kingdom.) 

If a plant is to grow, its photosynthesis during the day must build 
up enough organic matter to more than make up for the material 
lost by respiration not only during the day but also during the night. 
In other words, the crucial value for the continued existence of the 
plant is the compensation depth for the 24-hour period. This will 
obviously occur at a shallower depth than the values reported for 
experiments limited to the middle of the day. The compensation 
depth for the complete day ranges between 20 and 30 m during the 
summer in clear coastal water of the temperate oceans. In the winter 
and in less transparent water the compensation depth occurs cor- 
respondingly nearer the surface. No constructive growth is possible 
for diatoms or other pelagic plants below these levels. When we 
recall that the average depth of the ocean is more than 4000 m, it is 



240 



Light 



apparent that the zone in which productive growth of the crucially 
important phytoplankton can take place is a relative skimming of 
the surface. The photosynthesis in the upper few meters of the 
ocean and of lakes accounts for the main portion of the initial pro- 
duction of organic matter for the whole breadth, length, and depth 
of the water. 



Surface 

100m- 
160 - 

250 - 



500 - 
550 - 



800 - 



1,000 - 



Limit of growth for planktontc plants 
Greatest depth for benthic plants 

Daylight 0.001% of surface value 



Approximate limit for vision of fish 
Complete darkness for man 



Limit of response to day and night 
by Crustacea 

Photographic plate blackened 
after 80 minutes' exposure 



No perceptible light 
from the surface 



1,700 -P Plate not affected after 120 mfnutes' 
exposure 



4,000 
10,860 



- Average depth of ocean 

- Deepest recorded sounding in ocean 



Summary of limitations by light factor in aquatic environment, Values 
are for clearest water. 

A summary of the conditions of light in the aquatic environment 
and its critical limiting effects is given in Fig. 6.29. Here are indi- 
cated the maximum depths for the growth of phytoplankton and 
slightly deeper, of benthic plants. Since the mean illumination 
necessary for the vision of aquatic animals is so very much smaller, 



Aquatic Plants 241 

the depth limitation is at a much greater level. Animals can respond 
to the difference between day and night at somewhat greater depths. 
Below this level no perceptible light from the surface penetrates and 
the water is completely dark except for light provided by the lumi- 
nescent organs of deep-sea animals. Terms used in deep bodies of 
water for zones based on the light factor are as follows: 

Euphotic Zone: Sufficient light for photosynthesis. 

Disphotic Zone; Insufficient light for photosynthesis but sufficient light for 

animal responses. 
Aphotic Zone: No light of biological significance from the surface. 

The depth limits of these zones differ widely according to trans- 
parency. 

The discussion in this chapter has revealed the many important 
roles played by light in the world of life. Although there is gen- 
erally sufficient light on land for plants and animals, the special cir- 
cumstances of this factor exert a wide control over the activities of 
many terrestrial organisms. In the aquatic environment light is even 
more crucial since, in addition to its various periodicities, its rapid 
changes with depth impose serious limitations on the lives of both 
animals and plants. 



7 

Oxygen and 
Carbon Dioxide 



With this chapter we turn from the more mechanical or physical 
features of the environment to a consideration of some of the chemical 
factors. Two fundamental substances taking part in the chemical 
exchange between the organism and the environment are oxygen and 
carbon dioxide. These substances enter into the basic processes of 
photosynthesis and respiration, as may be indicated by the overall 
reactions in which C 6 H 12 O 6 is taken to represent the carbohydrate of 
the organism. 

Light Photosynthesis 

C0 2 + H 2 O ;=i C 6 H 12 O, + 2 

Respiration 

Oxygen and carbon dioxide thus stand in a reciprocal relation to each 
other as regards these fundamental reactions of life, and their 
abundance in the environment is of direct critical concern to the 
organisms of every habitat. In addition, these materials take part in 
subsidiary reactions involved in the ecological relations of plants and 
animals. Of particular ecological importance in this connection are 
the decomposition and transformation of organic matter, carried out 
primarily by microorganisms, since in these processes oxygen is con- 
sumed and carbon dioxide is released. 



OXYGEN 

Oxygen is needed by almost all organisms to make available the 
energy contained in organic food materials. The great majority of 
plants and animals use free oxygen from the air or from the water for 
the oxidation of organic substances; these are aerobic organisms. 

242 



Terrestrial Environment 243 

Anaerobic forms, on the other hand, get their energy by partial de- 
composition of organic matter without free oxygen. Anaerobic or- 
ganisms nevertheless depend upon the aerobic forms to produce the 
organic matter upon which they live. As we shall see, the abundance 
of oxygen in the environment may become critically low for aerobic 
organisms but it never becomes harmfully high for these forms. 
Natural concentrations of oxygen may, however, be seriously detri- 
mental to some anaerobes. 

Oxygen is present in the air as one of the gases that are physically 
mixed together, and it occurs in water in simple solution. Dissolved 
oxygen does not combine chemically with water itself but it does react 
with iron and other inorganic materials in the water. Points of con- 
trast in this respect with carbon dioxide will be discussed later in the 
chapter. Since oxygen is taken up and given off by life processes, 
its concentration in the environment can sometimes be appreciably, or 
even seriously, altered by the activities of the plant and animal in- 
habitants. Under these circumstances oxygen is a modifiable factor 
of the environment. 



Availability of Oxygen 

Terrestrial Environment. Oxygen constitutes 21 per cent of the 
atmosphere, and this value varies by less than 1 per cent the world 
over. Although plants and animals are continuously drawing upon 
the oxygen supply in the air, and plants are periodically adding to 
it, the concentration in the atmosphere is not changed appreciably 
by these life activities because of the great volume and mobility of 
the air medium. Most of the terrestrial environment is thus pro- 
vided with a uniform and adequate supply of oxygen. 

In two types of situations of importance in the terrestrial environ- 
ment a lack of oxygen exists at high altitudes and in the subsurface 
layers of the soil. At great elevations the concentration of oxygen is 
low because all gases have become rarer. The amount of reduction 
in oxygen with altitude is proportional to the reduction in total at- 
mospheric pressure, discussed in Chapter 2. Thus at an altitude of 
5500 m O 2 is only half as abundant as it is at sea level. The oxygen 
in soils drops from near the atmospheric value of 21 per cent at the 
surface to about 10 per cent within well-drained loams and to lower 
values in poorly aerated soils and in layers below the water table. 
Lack of air circulation results from very fine texture or from the 
flooding of the pore spaces. Under these circumstances the respira- 
tion of roots and of soil organisms, particularly those involved in the 



244 Oxygen and Carbon Dioxide 

decomposition of organic matter, reduces the oxygen supply faster 
than it can be replenished from the atmosphere. 

Aquatic Environment. The total amount of oxygen that water will 
hold at saturation varies with temperature, salinity, and pressure. 
Sample oxygen saturation values for fresh water and sea water at 
temperatures within the range of ecological interest are given in 
Table 17. When we realize that the 21 per cent of oxygen present 

TABLE 17 
CONCENTRATIONS OF OXYGEN AT SATURATION 

Fresh Water Sea Water 



Temp. C 


O 2 ppm. 
(ing/liter) 


O 2 cc/liter 


(salinity S6&) 
O 2 cc/liter 





14.7 


10.3 


8.0 


15 


10.3 


7.2 


5.8 


30 


8.3 


5.6 


4.5 



in the atmosphere is equivalent to 210 cc per liter, the contrast be- 
tween the amounts available in air and in water is brought into relief. 
There may be 25 times as much oxygen in a liter of air as in a liter 
of water. When the oxygen in the water is in equilibrium with 
that in the air, however, the pressure of the oxygen is the same in 
both media. 

From the foregoing it is clear that the world's main Reservoir of 
free oxygen is in the atmosphere. The chief source eft oxygen for 
the water environment is the oxygen which can be absorbed from 
the air, but a second source of supply in aquatic habitats is the oxygen 
released beneath the surface by the photosynthesis of submerged 
and planktonic plants. Water may lose oxygen by diffusion from its 
surface out into the atmosphere. Oxygen is also used up by the 
respiration of aquatic organisms and by the decomposition of organic 
matter in the water. Living organisms are thus seen to influence 
both the supply and the depletion of oxygen in natural bodies of 
water; oxygen is a modifiable factor in the aquatic environment but it 
is essentially an unmodifiable factor in the air. The actual amount 
of oxygen present at any one time and place in the water is the result 
of a balance between the processes of supply and depletion. Here is 
another situation in which an ecological factor is controlled by a 
dynamic equilibrium involving both physical and biological processes. 

(Three general situations may be discerned in regard to the supply 
of oxygen in the water environment: the surface stratum, the sub- 
surface zone, and the deep layers. The water at the surface tends 
to be in equilibrium with the air above it so that the oxygen value 



Aquatic Environment 245 

is at or near the saturation concentration for the existing conditions 
of temperature and salinity. The thickness of the surface stratum 
in which saturation equilibrium is found varies greatly according to 
the amount of turbulent mixing of the water in contact with the air. 

In the subsurface zone great variation in oxygen concentration may 
occur because the water does not have easy exchange with the at- 
mosphere and factors causing both decrease and increase in oxygen 
are present. Respiration and decomposition, tending to deplete 
the oxygen supply, occur at all levels, and photosynthesis, tending to 
increase the abundance of oxygen, takes place down to the com- 
pensation depth, that is, to the lower limit of the euphotic zone, as 
described in the previous chapter. /When large populations are 
respiring or large quantities of dead material are decomposing, the 
oxygen concentration is reduced to a low level. ^ In stagnant ponds 
and swamps choked with organic matter, and in rivers or other 
bodies of water receiving excessive amounts of sewage or other 
pollutants, the available free oxygen often becomes completely ex- 
hausted/ Under other circumstances supersaturation may occur in 
subsurface water layers. If water which was saturated during the 
winter becomes warmed as the season advances, more oxygen will be 
present than can be held in true solution at the higher temperatures. 
Similarly, when photosynthesis exceeds respiration at intermediate 
levels in the euphotic zone, oxygen may be released into the water 
faster than it can be carried away. As a result of these physical and 
biological influences, supersaturated values as high as 180 per cent 
in the sea and 300 per cent in inland lakes have been reported. 

Often cyclic fluctuations in oxygen content are observed in natural 
bodies of water. Seasonal changes in temperature and circulation 
coupled with differences in rates of photosynthesis, respiration, and 
decomposition result in changes both in the absolute amount of 
oxygen present in the water and in the degree of saturation. Under 
certain circumstances a pronounced diurnal fluctuation in oxygen 
concentration is found; this is known as an oxygen pulse. During the 
daylight hours photosynthesis tends to cause an increase in the 
amount of oxygen present, but, after the sun has set, the respiration 
of the aquatic organisms and the decomposition processes going on 
draw on the free oxygen in the water. The amplitude of the oxygen 
pulse may reach considerable proportions in quiet waters in which 
green plants are growing in abundance. Measurements in an Ohio 
lake, for example, revealed a diurnal fluctuation in oxygen concentra- 
tion extending from 6.7 ppm at 8:00 A.M. to 13.0 ppm at 5:00 P.M. 
(Fig. 7.1). 



246 Oxygen and Carbon Dioxide 

How does oxygen get to the bottom of the ocean? We have seen 
that the sources of oxygen are at or near the surface, but we know 
that many kinds of aerobic animals live at the bottom of some lakes 
and in the ocean abyss at depths of several miles beneath the surface. 
Although oxygen moves slowly through the water by direct diffusion, 
it can reach the deeper levels at an adequate rate only by the circula- 
tion of the water itself. 



6 pm 



12 mid. 



Sam 



ppm 
15 

fo 10 



"? 




3-5 



-10 



12 noon 5pm 




Free carbon dioxide f 

x 

>o--o (f 



Modified from Tressler, Tiffany, and Spencer, 1940 

FIG. 7.1. Diurnal cycle of temperature, oxygen, hydrogen ion (as pH), and car- 
bon dioxide in the surface waters of Buckeye Lake, Ohio, during August 12-13, 
1930. Negative values for CO 2 represent amount of CO 2 required to make water 
neutral to phenolphthalein. 

Oxygen-rich surface water is carried downward by wind stirring, 
eddy conduction, and mass sinking. Wind stirring is very effective 
in aerating the upper layers of lakes and of the ocean at times when 
the water circulates freely. Whenever a water body is covered by 
ice, no turbulence caused by wind or waves is possible. At other 
seasons stirring is greatly curtailed whenever the water is stratified in 
respect to densityas a result of salinity differences occurring at any 
time of year, or as a result of a thermal gradient arising primarily 
in summer. During periods of pronounced stratification the circula- 
tion produced by wind and waves is limited to the layers above the 
density discontinuity (Fig. 7.2). When stratification has been de- 



Aquatic Environment 247 

stroyed, wind stirring may extend sufficiently deep to reach the bottom 
of shallow lakes and coastal waters, and thus to replenish the oxygen 
supply. There is a limit, however, to the depth to which turbulence 
caused by wind is effective. In the open ocean, where the highest 
winds and largest waves occur, little influence of wave action is felt 
below 100 m. 

Direction of wind 



r -*" Shearing ~* -" Plane -* 

y , ~^-_ "^vr "^ ^ 

f__ ^^~ "*^n Return ^-, currents*-* 





Thermocline 




Fir,. 7.2. Wind-driven currents in a lake with a pronounced thermocline. Circu- 
lation is limited to the epilimnion. (By permission from Limnology by Welch, 
Copyright 1952, McGraw-Hill Book Co.) 

Oxygen may be carried to depths beyond the limit of effective wave 
action by eddy conduction. Wherever differential movement of cur- 
rents occurs, eddies are generated along the current margins, and 
these cause units of water to be exchanged (Fig. 7.3). The exchange 
will bring about a transfer of any characteristics of the water that 
differ in the adjacent water masses. The rate of transfer depends upon 



SURFACE WATER 

Eddy transfer 

O-O-O-O-O-O 



CURRENT 



>-O-O-O-0-O 

Eddy transfer 
DEEP WATER 



FIG. 7.3. Diagrammatic vertical section of a body of water, indicating vertical 

transfer by eddies across the current boundaries. Horizontal eddy conduction 

similarly takes place at the lateral margins of the current. 



248 Oxygen and Carbon Dioxide 

(a) the rate of mixing of water units and (b) the difference in con- 
centration of each characteristic. The rate of mixing, or austausch 
coefficient, is a property of the water under the existing conditions of 
turbulence, and it affects all characteristics that are being exchanged 
between water masses, At the same time that oxygen is being trans- 
ferred, heat may also be moved downward, phosphate moved upward, 
and other exchanges carried out by the same eddy action between 
the water masses. This is an illustration of the principle of common 
transport: a current that transfers one property of the medium also 
transfers other properties at the same time (Redfield, 1941). 

Eddy conduction is a process by which materials are transferred at 
right angles to the direction of current. The transfer may take place 
vertically, as in the situation just described, horizontally, or in some 
other direction. The downward transfer of oxygen by ecldy conduc- 
tion is slow but it is commonly a thousand times greater than the 
diffusion rate. Although eddy conduction helps furnish oxygen to 
water layers at middepths wherever currents of different speeds or 
directions are flowing, it cannot supply an adequate amount of this 
essential material to the deepest parts of lakes or of the ocean. 

Fortunately for the abyssal animals oxygen can reach them by 
another method, namely, the mass sinking or deep circulation of the 
water. In the typical deep lake of the temperate region water 
masses charged with oxygen at the surface sink to the bottom during 
the spring and fall overturns, as described in relation to the tempera- 
ture cycle. As a result of the extensive circulation of the lake at these 
periods the water may become turbid and formerly this was often 
noticeable in poorly filtered water supply systems. When muddy 
water issued from the taps, the residents would nod knowingly and 
say, "The pond is working." The limnologist, however, refers to 
the periods of overturn that bring a fresh supply of oxygen to the 
deeper layers after winter and summer stagnation as occasions when 
"the pond takes a breath." 

An example of the seasonal changes in oxygen at various depths in 
a temperate lake is taken from the classic work of Birge and Juday 
(1914) (Fig. 7.4). During the period of this representative study 
oxygen was abundant at all depths after the spring overturn but a 
rapid depletion began in May following thermal stratification^ Oxy- 
gen was almost completely absent at depths greater than 15 m during 
July, August, and September. The fall overturn in mid-October 
caused a sudden mixing and equalization of oxygen at all levels; in 
succeeding weeks increasing amounts of oxygen were supplied 
throughout the lake by deep circulation. In certain very deep lakes 



Aquatic Environment 249 

the seasonal circulation does not reach the bottom, with the result 
that the deepest layers may be completely devoid of oxygen through- 
out the year. 

In the temperate regions of the ocean the thermal stability built up 
during the summer is similarly broken down in the autumn. Turbu- 
lence produced by wind and waves then carries oxygen-rich water 
downward, and this deep stirring continues all winter. The down- 
ward extent of the winter mixing is sufficient to reach the bottom in 
coastal areas but its effect does not go below the permanent thermo- 
cline of the open sea (Fig. 5.4). Seasonal changes in temperature 
and wind action thus do not provide for the delivery of oxygenated 
water to the great depths of the ocean. 




May June July Aug. Sept. Oct. Nov. 

FIG. 7.4. Changes in oxygen during the warm part of the year at the indicated 
depths in Lake Mendota, Wisconsin. ( Needham and Lloyd, 1937, after Birge and 

Juday. ) 

Oxygen is supplied to the ocean abyss by deep permanent currents 
that originate near the surface in high latitudes, sink to intermediate 
levels or to the bottom, and flow for thousands of miles toward and 
beyond the equator (Fig. 7.5). This deep flow of water is dynam- 
ically integrated with the horizontal current systems with the result 
that the vertical movements of water follow a complicated pattern 
(Sverdrup et al, 1942). As a simplified generality, however, the 



250 Oxygen and Carbon Dioxide 

cold water that sinks to great depths may be understood eventually 
to find its way to the surface again in the central part of the ocean, 
and a return flow of water from the equatorial regions toward the 
poles is brought about as a part of the horizontal circulation of the 
upper water masses. 




5000 

70 S 60 50 40 30 20 10 S N 10 20 30 40 50 60N 

FIG. 7.5. Cross section of the Atlantic Ocean, showing the distribution of oxygen 
in cubic centimeters per liter. The north-south and vertical components of the 
oceanic currents are indicated by the arrows. (Modified from Svcrclrup et al., 

1942, after Wiist.) 

Sea water near the surface in tropical regions contains about 4 cc 
of oxygen per liter, but as it becomes colder at higher latitudes, 
oxygen is absorbed from the atmosphere and produced by photo- 
synthesis until a concentration of about 8 cc per liter is attained. 
When the water masses leave the surface and sink toward the bottom, 
depletion of oxygen begins as a result of respiration and decomposi- 
tion, but as much as 5 cc O 2 per liter remains when the water reaches 
the ocean abyss. Animals living at the bottom of the sea thus 
receive oxygen via a direct flow of water from the enriched polar 
seas, but this is perhaps the slowest delivery service in the world. 
Calculations based on the rate of consumption of oxygen and also on 
the relative proportion of CM in deep water indicate that movement 
of Arctic surface water to deep layers at midlatitudes may require a 
thousand years or more. 

The concentration of oxygen at any point in deep water depends 
upon (1) the amount that the water contained when it left the 
surface, (2) the rate at which oxygen is used up en route by respira- 
tion and decomposition, and (3) the time that has elapsed. Mini- 
mum values occur at positions in the ocean at which the combined 
effect of the foregoing factors has reduced the oxygen to the greatest 
extent. The lowest oxygen values are not found at the bottom but 
at certain intermediate regions, as indicated in Fig. 7.5. In no part 



Oxidation-Reduction Potential 251 

of the open Atlantic Ocean is the oxygen completely used up, and 
minimum values are generally above 3 cc per liter. In the Pacific 
Ocean, however, oxygen is reduced to zero in certain oxygen minimum 
layers, as for example, at depths between 300 and 1300 m off the 
coast of lower California. 

Other instances in the marine environment of the complete lack 
of oxygen, or of very low concentrations, are found only in special 
situations in which circulation is largely or entirely cut off and de- 
composing organic matter has accumulated. The classic example 
of this condition is in the free water of the Black Sea, in which no 
measurable oxygen is found from a depth of about 150 m to the 
bottom at 2200 m. Vertical stirring is prevented by a layer of fresh 
water at the surface supplied by the discharge of the Danube and 
other large rivers entering the Sea, and deep exchange with outside 
water is stopped by the shallow entrance at the Bosporus. Organic 
particles carried in with the river water use up the oxygen as they 
sink to deeper layers, and anaerobic decomposition produces great 
quantities of hydrogen sulphide. Similiar conditions on a smaller 
scale are found within deep fjords with shallow sills and within other 
estuaries in which the circulation does not supply the oxygen as fast 
as it is used up (Fig. 7.6). The bottom material of the marine en- 
vironment is also likely to develop a shortage of oxygen unless it is 
sufficiently coarse to allow good movement of water through it. 
Fine muds containing decomposing matter contain little or no 
oxygen at depths of more than a few centimeters from their surfaces. 

Oxidation-Reduction Potential. Another aspect of oxygen avail- 
ability in the aquatic environment, but less well known, is the oxida- 
tion-reduction potential, or redox potential. The redox potential 
(Eh), measured in volts, is an expression of the electropositivity or 
negativity of a substance in solution as referred to a hydrogen stand- 
ard. A positive redox potential indicates a condition tending to cause 
oxidation, and a negative potential indicates the reverse condition, 
tending to bring about reduction. As used to describe an aquatic 
habitat, redox is the overall summation of the redoxes of all the solutes 
present in the water. Redox potentials have two general properties 
that must be considered: intensity or Eh as defined above; and ca- 
pacity or poising, which is analogous to the buffering of a pH system. 
A high capacity reflects the tendency of the system to retain its in- 
tensity despite minor changes in its constituents. 

In well-aerated natural waters oxygen concentration usually gov- 
erns the redox potential and produces positive values. However, fer- 
rous complexes or oxygen shortage may allow various inorganic and 



252 Oxygen and Carbon Dioxide 

organic ions and molecules to set more negative values. Such low 
redox systems represent reducing agents that would tend to combine 
with any introduced oxygen. An aerobic organism, needing oxygen 
for its respiration, would find such competition exceedingly difficult, 
if not insuperable, if the redox system were well poised. Facultative 
and obligative anaerobes, however, have no such difficulty and are 
able to tolerate low redox values. 





FIG. 7.6. (A) Basin with local formation of heavier water and deep outflow 
across the sill, as in the Mediterranean Sea, the Red Sea, and the Inner Gulf of 
California. ( B ) Basin with surface outflow of lighter water and occasional inflow 
of denser water across the sill, as in the Black Sea, Baltic Sea, and many fjords. 
(From The Oceans by Svedrup et al, 1942, Copyright Prentice-Hall, N. Y.) 

Zonation of organisms according to redox potentials has been shown, 
especially in sediments, but whether the oxygen concentration or the 
redox potential is the determining factor is hard to ascertain since the 
latter generally depends chiefly upon the former. In marine muds 
the redox potential decreases with depth in the sediment with the 
most rapid change occurring in the few uppermost centimeters. The 
abundance of aerobic organisms correspondingly drops off sharply, 
and at increasing depths the anaerobes become relatively more 



Terrestrial Environment 253 

abundant. The type of bottom fauna in certain lakes has also been 
found to be related to the redox potential. In lakes in which the 
water above the deepest mud had an Eh above 0,4 volt, the mud 
itself supported a pure or mixed fauna characterized by midge larvae 
of the genus Tanytarsus, whereas in lakes with an Eh below 0.3 volt 
a true Chironomus fauna occurred (Hutchinson et al., 1939). For a 
further discussion of the ecological effects of redox potentials the 
reader is referred to ZoBell (1946); it is obvious that the influence of 
this aspect of the oxygen factor is due for closer scrutiny in the future. 

Effects of Oxygen Availability 

Terrestrial Environment. We may now examine the ecological 
effects of the varying amounts of oxygen present in different habitats. 
Since oxygen is amply abundant in the lower portions of the earth's 
atmosphere, this factor has no important limiting action in most ter- 
restrial situations. The reduction in the partial pressure of oxygen 
at high elevations imposes a restriction on the altitudinal distribution 
of organisms having a high oxygen requirement. No mammals can 
live permanently at altitudes at which the partial pressure of oxygen 
is less than about 45 per cent of its value at sea level. Oxygen short- 
age as well as the thinness of air imposes a similar ultimate limit on 
the altitude at which birds can live. The altitudinal range for most 
types of lower animals and for plants is limited by low temperatures 
or by other ecological factors long before the diminished oxygen con- 
centration becomes a serious handicap. 

Organisms requiring free oxygen are excluded from soils and 
ground debris within which the circulation of air is inadequate. 
The lack of oxygen in poorly aerated soils becomes seriously harmful 
to the roots of most plants at concentrations below about 10 per cent. 
Root penetration for most of the vegetation is stopped by soil layers 
containing less than 3 per cent oxygen and also by the water table 
since even less oxygen is available in the ground water. If areas with 
established vegetation are subjected to prolonged flooding, caused for 
example by the construction of beaver dams, most of the plants will 
be "drowned" or killed by the cutting off of root aeration. 

The roots of some plants, however, possess special adaptations en- 
abling them to grow in soil layers devoid of oxygen. In some types, 
including particularly the herbaceous hydrophytes, air passages within 
the stem and major roots allow oxygen to be brought from the 
atmosphere, or possibly from internal sources in the upper part of the 
plant, and delivered to the lower extensions of the root system. In 



254 Oxygen and Carbon Dioxide 

the black mangrove, common along the margins of Florida lagoons, 
root branches, known as pneumatophores, grow vertically upward 
through the mud and water until they project into the air, where they 
have ready access to oxygen. Other special features of plant growth 
found in permanent swamps are the "knees" and enlarged trunk bases 
displayed by such trees as the bald cypress (Fig. 7.7). The knees 




Photo U. S. Forest Service 

FIG. 7.7. Enlarged trunks and "knees" of cypress trees growing in permanent 
swamp conditions in the Francis Marion National Forest, South Carolina. 

were formerly believed to provide aeration for the roots, but evi- 
dence now indicates that these root excrescences and the enlarged 
trunks represent excessive cambial growth resulting from the com- 
bination of abundant water and oxygen supply for the tissue at the 
water line. The roots of a few vascular plants without observable 
adaptation for securing outside oxygen, such as the willow, are able 
to remain active during prolonged flooding; the root tissues in these 
exceptional instances are presumed to be capable of anaerobic 
existence. 

Most cryptogamic plants and soil animals depend upon aerobic 
respiration and hence are excluded from soil layers devoid of oxygen. 
A few of the higher forms, such as the earthworm, may be able to 



Aquatic Environment 255 

do without oxygen for short periods, as when the soil is rain soaked, 
but for the most part earthworms migrate to the surface when oxygen 
becomes depleted. Many organisms encyst, or go into a dormant 
condition, when oxygen becomes too scarce for the species concerned. 
However, some of the microorganisms inhabiting the soil can follow 
an anaerobic existence (Waksman, 1932). Included are members of 
the genera Clostridium and Rhizobhnn that are important as nitrogen- 
fixing bacteria; Clostridium tetani is of particular concern to man as 
the microbe that causes lockjaw. We realize then that the highly 
variable conditions of oxygen availability often exert a critical control 
over the distribution and activity of both aerobic and anaerobic 
organisms involved in the ecology of the soil. 

Aquatic Environment. We have seen that the supply of oxygen 
in the aquatic environment varies over the complete range from super- 
saturation to total exhaustion, but that, even when saturated, water 
contains a much lower concentration of oxygen than the atmosphere. 
Some fresh-water organisms have hit upon certain dodges by which 
they can live in the aquatic medium and still breathe air. Many 
water bugs, such as Notonecta, and certain beetles come to the surface, 
entrap bubbles of air beneath their wing cases, and swim down with a 
supply of oxygen that will last for a period of time. Other animals, 
like the water scorpion Ranatra ( Fig. 6.12), the mosquito larva, and the 
rat-tailed maggot, possess breathing tubes that extend to the surface. 
Some kinds of snails periodically refill their lungs with air at the 
surface, whereas other water animals are able to bore into the stems 
or roots of emergent hydrophytes and obtain oxygen from the internal 
air spaces of the plants. 

Most aquatic organisms, however, must get along with the rela- 
tively meager supply of oxygen dissolved in the water. Let us con- 
sider the ecological conditions in well-aerated water before taking up 
the effects of severe oxygen depletion. Although saturated water 
contains only a small fraction of the amount of oxygen in an equal 
volume of the atmosphere, the partial pressure of oxygen in the water 
is equal to that in the air with which it is in equilibrium. Accord- 
ingly, the tendency of the oxygen in water to cross the respiratory 
membranes of organisms is just as great as it is in air. The lower 
concentration of oxygen in water simply means that less is in reserve. 
The relatively small amount of oxygen in the water is adequate for 
aquatic organisms if the medium in contact with the absorbing mem- 
branes is rapidly replenished. 

Inhabitants of the water environment display a wide variety of 
adaptations that serve to obtain a sufficient supply of oxygen from the 



256 Oxygen and Carbon Dioxide 

medium. Sessile animals cause water to flow through their respira- 
tory chambers or wave their respiratory organs about in the surround- 
ing water. The echiuroid worms keep a current of water flowing 
through their burrows in the sand when the tide is in by rhythmic 
contraction of their bodies, but reduced metabolism allows them to 
withstand periods of 18 hours when their tide flats are out of water. 
More active animals, such as most fish and higher crustaceans, possess 
special mechanisms for pumping water continuously over their gills. 
Certain fish, however, reverse the process and produce a flow over 
the gills by swimming rapidly through the water with their mouths 
open. Some species, as, for example, the mackerel, have come to 
rely entirely on this latter method, and in the course of evolution the 
gills and opercular muscles have become reduced, with the result 
that these fish cannot obtain sufficient oxygen while remaining sta- 
tionary. If a mackerel is confined in a small space, or prevented from 
swimming rapidly, it will die of asphixiation even though the sur- 
rounding water is saturated with oxygen ( Hall, 1930 ) . 

In view of the foregoing one might expect that in those aquatic 
habitats in which the water is not saturated with oxygen, the lack of 
this vital substance would immediately have serious consequences. 
However, many aerobic aquatic organisms are quite able to survive 
with concentrations of oxygen much below normal pressures. The 
metabolism of some species is so low that a small oxygen supply is 
sufficient. Certain species possess special respiratory pigments or 
other physiological adaptations that aid in the absorption of oxygen 
at very low partial pressures. Chironomid larvae, inhabiting the 
muddy bottoms of ponds, are furnished with a type of hemoglobin 
that becomes 95 per cent saturated under a partial pressure of oxygen 
of only 10 mm Hg, whereas mammalian hemoglobin is less than 1 per 
cent saturated at this pressure. These insects are also capable of a 
remarkably simple anaerobic metabolism in which the lactic acid 
formed from glycogen breakdown is excreted, thus eliminating this 
toxic compound rather than oxidizing it as most animals do. 

The relation of oxygen consumption by animals to the tension of 
oxygen in the environment follows two general patterns: (1) the non- 
regulatory type in which consumption is highly dependent upon ten- 
sion, as seen in certain annelids and arthropods (e.g., Nereis, Hornarus, 
Limulus, and Callinectes), and (2) the regulatory type in which con- 
sumption is independent of oxygen pressure over a wide range, as 
seen in certain crustaceans and mollusks (e.g., Astacus, Carcinus, 
Aplysia, and Eledone). In the latter type the harmful effects of in- 
sufficient oxygen supply appear rather suddenly when the oxygen 



Aquatic Environment 257 

drops below the critical tension for the species concerned. The 
critical oxygen tensions for a large number of animals, a further dis- 
cussion of adaptations to low oxygen supply, and a consideration of 
physiological relations to the oxygen factor in general are to be found 
inProsser (1950, Ch. 8). 

It is difficult to determine the minimum concentrations of oxygen 
required by various aquatic animals. The value is influenced by 
temperature, pH y and other modifying factors, as well as by the 
degree and rate of acclimatization to low oxygen tensions. In the 
hypolimnion of lakes, where oxygen is likely to be depleted, the tem- 
perature is low, with consequent reduction in the animal's metabolism 
and in its need for oxygen. Organisms require more oxygen when 
in an active condition than when quiescent; some species enter a 
dormant condition in which they can withstand varying degrees of 
oxygen deprivation for varying lengths of time. The ecologist thus 
recognizes that the animals caught in a water layer containing little 
or no oxygen are not necessarily able to live permanently under the 
observed conditions: they may be in a dormant condition for a 
limited period, or they may have made a temporary excursion into the 
poorly aerated zone. Nevertheless we have obtained a knowledge of 
the approximate oxygen requirements of many common aquatic 
animals, and even among completely aerobic forms great differences 
in the minimum values tolerated have been revealed. At one extreme 
are those like the toadfish that seems to be able to survive if any de- 
tectible amount of oxygen exists in the water. Carp and other fish 
adapted to live in muddy ponds often show no harmful effects until 
oxygen has been reduced to 0.3 cc per liter or even to 0.1 cc per liter. 
At the other extreme are active fish like the mackerel that requires 
3.6 cc per liter (70 mm Hg) at 24C (Hall, 1930). 

In those ponds, lakes, and fjords, in which the oxygen concentration 
of the lower levels is reduced to zero during the summer ( and in some 
instances also during the winter) the inhabitants must either migrate 
out of the oxygenless zones or enter upon some sort of anaerobic 
existence, if they are to survive. Copepods and other types of zoo- 
plankton are known to leave the hypolimnion of lakes as summer stag- 
nation begins. Fish and the motile bottom fauna undertake similar 
seasonal vertical migrations. Many animals and plants with no 
means of escape die off in large numbers after the exhaustion of the 
oxygen supply. However, certain species of worms, crustaceans, 
insect larvae, mollusks, and other invertebrates are known to remain 
in the bottoms of lakes and to survive for periods of many weeks each 
year with no detectible oxygen present in the bottom water. A 



258 Oxygen and Carbon Dioxide 

further discussion of the types of metabolism displayed by inverte- 
brate animals capable of anaerobiosis is given by von Brand (1946). 

When the supply of oxygen in a confined aquatic habitat becomes 
reduced below the level of toleration, the lethal effects sometimes 
appear quite suddenly. We occasionally see reports that all the fish 
in a pond have been found dead on one morning whereas the day be- 
fore they had appeared to be in a perfectly healthy condition. Such a 
situation occurs most frequently after a hot calm spell in summer. 
Because of the high temperature the amount of oxygen that the water 
can hold is low and the rates of respiration by the pond organisms and 
of the decomposition of organic materials in the water are high. In 
calm weather no effective stirring takes place to replenish the sub- 
surface water levels. As a result of this combination of circumstances 
the oxygen minimum reached by the diurnal pulse at the end of each 
night becomes progressively lower. Early one morning the oxygen 
concentration drops below the toleration point, and the whole popula- 
tion of fish in the pond is suffocated. 

In aquatic habitats permanently and completely devoid of oxygen 
no aerobic organisms can live. Included in this category are: the 
deep waters of certain lakes, the Black Sea, and certain arms of the 
ocean with insufficient circulation; the subsurface layers of mud 
bottoms; and many heavily polluted waters. Such places can be per- 
manently inhabited only by forms that are able to function indefinitely 
on an anaerobic basis. The population of these habitats consists 
chiefly of anaerobic Protozoa and bacteria. Many of these obligate 
anaerobes are killed by the presence of oxygen. This fact is put to 
practical use in the employment of thorough aeration as one step in 
the purification of water supplies. Certain higher animals, such as 
slime worms (Tubificidae), pea clams (Pisidium), and insect larvae 
(Corethra and Chironomus), are found in the bottoms of very deep 
lakes where oxygen is very low or absent. 

The organisms that inhabit the oxygenless water layers carry out a 
partial decomposition of the organic matter filtering down from above, 
and frequently release large quantities of hydrogen sulphide in the 
process. In the Black Sea the permanent absence of oxygen in the 
deeper water layer brings the vertical distribution of aerobic benthic 
animals to an end at depths varying between 115 m and 165 m accord- 
ing to the locality. The plankton is similarly limited to the upper 125 
m in the central part of the sea but may extend to 175 m in marginal 
areas where the surface water is stirred more deeply. Lack of oxygen 
and abundance of poisonous hydrogen sulphide exclude all life except 
anaerobic microorganisms from the lower 2000 m of the Black Sea. 



Carbon Dioxide 259 

In the foregoing review of oxygen we have seen that this life- 
giving material is sufficiently abundant in most major habitats, but 
that it becomes critically scarce in certain special situations. Most 
land organisms find enough oxygen in their habitats, and the great 
majority of marine animals are so adapted that the amount of oxygen 
in sea water ordinarily meets their requirements, although its con- 
centration is far less than in the atmosphere. As green plants photo- 
synthesize and grow, they produce more oxygen than they consume, 
and hence run short of oxygen only in rare instances. But we have 
seen that in fresh water and under certain conditions on land and in 
the sea, available oxygen may become seriously reduced, with the 
result that whole populations may be killed unless they have means 
of escape, or are capable of anaerobic existence. The distribution of 
aerobic plants and animals is sharply curtailed at the margins of 
habitats permanently devoid of oxygen, and within these areas the 
relatively few groups of completely anaerobic organisms hold un- 
disputed domain. 

CARBON DIOXIDE 

A consideration of carbon dioxide as an ecological factor is of vital 
concern first of all because it is one of the essential ingredients of 
the photosynthetic reaction. This material thus provides the carbon 
source necessary for the growth of all green plants and indirectly for 
all other organisms. Carbon dioxide also influences other features of 
the environment. Quite in contrast with oxygen, carbon dioxide 
combines chemically with the water medium itself, and forms car- 
bonic acid. Through this reaction it influences the hydrogen ion 
concentration (measured as pH), and it forms compounds with 
calcium and other elements of ecological importance. Carbon di- 
oxide affects the respiration of animals and, in combination with 
calcium, takes part in the formation of their bones and shells. 

When animals and plants respire, organic matter is oxidized and 
carbon dioxide is produced. This process is the reverse of photo- 
synthesis as indicated in the equations at the beginning of the chapter. 
Carbon dioxide is also released when dead organic matter is decom- 
posed by the oxidizing or fermenting activities of microorganisms. 
On the other hand, carbon dioxide is withdrawn from the environment 
by the growth of green plants and by lime secretion. Thus, carbon 
dioxide, like oxygen, enters the living complex in an absolutely vital 
way. Since the abundance of carbon dioxide in many situations is 
strongly affected by living organisms, we may definitely list this ma- 



260 Oxygen and Carbon Dioxide 

terial as a modifiable property of the environment. Because of the 
reciprocal relation between carbon dioxide and oxygen in the funda- 
mental reactions of nature mentioned above, we generally find that 
wherever the supply of one of these materials has been depleted the 
concentration of the other has been increased. 



Carbon Dioxide in the Terrestrial Environment 

It is a striking fact that carbon dioxide a material so essential in 
the ecological relations of organisms is present in the earth's atmos- 
phere in an extremely small amount; it constitutes only 0.03 per cent 
of the air medium. Carbon dioxide is thus only about % o as 
abundant in the air as oxygen. Because of the great mobility of the 
atmosphere, however, carbon dioxide is well distributed, and its low 
concentration is generally sufficient for at least a moderate amount 
of photosynthesis the earth over. Possibly the reduction in carbon 
dioxide at great altitudes would seriously curtail the photosynthetic 
rate of plants, but low temperature or poor soil conditions are usually 
far more important in limiting plant growth in the mountains, 



0.200 



Carbon dioxide 0.010 volume percent 




400 600 800 1000 

Light intensity, foot-candles 

Fie. 7.8. Relation between rate of photosynthesis of wheat plants and light in- 
tensity at normal atmospheric, at decreased and at increased concentrations of car- 
bon dioxide. (From Meyer and Anderson, 1939, after Hoover et al., Smithsonian 

Misc. Coll, 1933.) 



COt in the Terrestrial Environment 261 

The normal amount of carbon dioxide available in the atmosphere, 
although allowing adequate plant growth under usual circumstances 
at ordinary altitudes, is insufficient for photosynthesis to attain rates 
that would be possible under favorable natural conditions if the con- 
centration of carbon dioxide were higher. During conditions of low 
illumination the light factor ordinarily controls the rate of photo- 
synthesis, but under high illumination the small amount of carbon 
dioxide in the air acts as the limiting factor. This situation is illus- 
trated in Fig. 7.8 in which rate of photosynthesis of wheat plants in 
relation to illumination is plotted under conditions of normal, in- 
creased, and decreased tensions of carbon dioxide. In this experiment 
under a normal carbon dioxide tension of 0.037 per cent the rate of 
photosynthesis (as measured by utilization of carbon dioxide) in- 
creases with increasing illumination up to 1000 ft-c. At higher il- 
luminations, beyond the limits of the graph, little further increase in 
photosynthetic rate took place, as the availability of carbon dioxide 
became limiting. With carbon dioxide at 0.010 per cent the limiting 
action of carbon dioxide availability became complete at 400 ft-c, 
whereas an increase of the carbon dioxide concentration to 0.111 per 
cent resulted in an augmentation of photosynthesis at all illuminations, 
with the curve still steeply rising at 1000 ft-c. Practical advantage of 
this situation can sometimes be taken by artifically increasing the car- 
bon dioxide in sealed greenhouses. Improved growth of certain 
plants is found for tensions of carbon dioxide of three to twenty times 
the normal amount, but higher concentrations are harmful. 

In nature some instances of local significant increases in atmospheric 
carbon dioxide have been reported. In certain volcanic areas carbon 
dioxide escapes from fissures in the rocks and, being heavier than 
air, forms a layer near the ground. From these "death valleys," such 
as the one in Java mentioned earlier, all animal life is excluded. The 
carbon dioxide in soil air generally is increased as a result of decom- 
position and respiration beneath the ground surface under conditions 
of poor aeration. The air under dense vegetation may exhibit values 
considerably above normal, and the air just above the soil of certain 
cultivated fields has been found to show a tenfold increase in carbon 
dioxide content, but no evidence is available as to any ecological ef- 
fects that may have been produced. In an experiment conducted in 
Georgia, however, the carbon dioxide in the atmosphere was found 
to be 25 per cent higher on certain foggy days, and during these pe- 
riods the photosynthesis of plants under observation was increased 
to as much as seven times the normal value (Wilson, 1948). 

Our atmosphere receives carbon dioxide from geological and in- 



262 Oxygen and Carbon Dioxide 

dustrial sources. The supply is increased by carbon dioxide released 
from the ocean and from respiration and decay. Carbon dioxide is 
withdrawn from the atmosphere by the photosynthesis of the vegeta- 
tion, The existing balance between supply and consumption provides 
a very low concentration of carbon dioxide in the air, but, if the 
minute amount of this essential gas were not present, all life on land 
would come to a stop. The lives of terrestrial organisms thus depend 
upon a very slender thread. In former geologic epochs the abun- 
dance of atmospheric carbon dioxide may have been considerably 
greater or smaller with the consequent possibility of significantly in- 
creased or decreased photosynthesis. At present we realize that car- 
bon dioxide is a general limiting factor in the sense that plant growth 
might go on at a higher pace in the world as a whole if our atmosphere 
contained more of this gas. However, since carbon dioxide is essen- 
tially uniform over the earth's surface, it does not cause significant 
differences in the distribution and growth of the terrestrial vegetation. 

Carbon Dioxide in the Aquatic Environment 

The amount of carbon dioxide present in the medium is crucially 
important for aquatic organisms just as it is for terrestrial forms, but 
the circumstances of the occurrence of carbon dioxide in the water are 
much more complex. Carbon dioxide is readily soluble in water, but 
since the concentration of carbon dioxide in the atmosphere is so low 
(0.3 cc per liter), water in equilibrium with it will hold only about 
0.5 cc per liter free CO 2 at 0C, or 0.2 cc per liter at 24C, in simple 
solution. Actually, the total CO 2 in most natural waters is consider- 
ably larger because additional amounts are present in the form of 
carbonate and bicarbonate ions. Sea water at a salinity of 35% 
normally contains a total of about 47 cc CO 2 per liter, or roughly 150 
times the concentration in the air. Taking into account the relative 
volumes of the ocean and of the atmosphere, it has been estimated 
that more than 50 times as much CO 2 exists in the seas as in the air 
( Rubey, 1951 ) . The ocean is thus the great reservoir for the world's 
supply of available CO 2 and this marine source tends to regulate the 
amount in the air just the reverse of the situation with oxygen. 

Reactions of Carbon Dioxide in Water. Dissolved CO 2 combines 
with H 2 O to form carbonic acid, and this dissociates as follows: 

C0 2 + H 2 ^ H 2 C0 3 ^ H+ + HCOr ^ H+ + CO,- 

The amount of CO 2 in simple solution plus that in the form of H 2 CO 3 
is called the free CO 2 . The amount of the CO 2 in the bicarbonate 



Reactions of Carbon Dioxide in Water 263 

ions (HCO 3 ~) and in the carbonate ions (CO 3 = ) is called the com- 
bined CO 2 . If strong acid is added, the combined CO 2 will be con- 
verted to the free form. The amount of acid required to accomplish 
this is a measure of the alkalinity, that is, of the amount of anions of 
weak acids (chiefly HCO 3 " and CO 3 = ) in the water, and also of the 
cations balanced against them. The alkalinity of sea water bears a 
fairly constant relation to the chlorinity and hence to the salinity. 
The relation of the equilibria of CO 2 to the alkalinity of sea water is 
discussed in further detail by Rakestraw (1950); general accounts of 
the reactions of CO 2 are given by Harvey (1945) for sea water and 
by Ruttner (1953) for fresh water. 

When H 2 CO 3 dissociates, it releases hydrogen ions (or more prop- 
erly, hydronium ions, H 3 O). These affect the pH, which is the 
negative logarithm of the hydronium ion activity. The pH is a meas- 
ure of the degree to which the water is acid ( pH below 7 ) or alkaline 
(pH above 7). If the equilibrium which is established in the water 
is near neutrality (pH = 7), most of the CO 2 will be present as 
HCO 3 ~ ion. At high pH values more CO 2 is present as CO 3 ion, 
and at low pH values more is present in the free condition. Thus 
pH and the relative distribution of CO 2 in its three components are 
mutually interdependent, as indicated in Fig. 7.9. Addition or re- 



100 




7 8 9 10 11 12 

pH 

FIG. 7.9. Percentage of total carbon dioxide in each of its three forms in water as 
a function of hydrogen ion concentration. (Emerson and Green, 1938.') 

moval of CO 2 will affect pH; conversely, any other factor changing 
the pH will affect the CO 2 equilibrium (Dye, 1952). 

The bicarbonate and carbonate ions and other anions of weak acids 
form a buffer system which tends to resist changes in pH. The buffer 
capacity of the water is determined by the abundance of these anions, 
and it is therefore also directly related to the alkalinity. Sea water 
and hard fresh water are relatively highly buffered natural media. 



264 Oxygen and Carbon Dioxide 

As a result of buffering, the surface waters of the open ocean rarely 
depart from a pH of 8.0 to 8.4, the deeper waters being generally 
confined to a pH of 7.4 to 7.9. In the littoral zone, and particularly 
in salt ponds and tide pools, the range is greater. The pH in marine 
muds may drop below 7.0 or rise above 8.5, and in dense growths of 
sea weeds the pH may reach 9.0. Sea water and hard fresh water are 
always on the basic side of neutrality. The pH of different fresh 
waters varies from values below 3 to values higher than 10, although 
the range of most streams and lakes is confined between pH 6.5 and 
8.5. In soft waters with small amounts of combined carbon dioxide 
and little buffering, the pH value may fluctuate widely within short 
periods. 

The respiration of free-living aquatic plants and animals and the 
decomposition of organic matter by microorganisms in the water add 
to the carbon dioxide supply of the water medium. The photosyn- 
thesis of the plants, on the other hand, tends to reduce the amount of 
available carbon dioxide present. In poorly buffered ponds and lakes 
variation in the relative rates of carbon dioxide production and con- 
sumption produces significant changes in the abundance of free car- 
bon dioxide and in the pH of the water. If these water bodies contain 
a good growth of green plants, the fluctuation in carbon dioxide and 
in pH displays a diurnal cycle as a result of the fact that photosyn- 
thesis regularly exceeds respiration during daylight hours. This car- 
bon dioxide and pH pulse is generally coordinated with, but recipro- 
cal to, the oxygen pulse described above. In the Ohio lake observa- 
tion illustrated in Fig. 7.1 the pH rose from 8 to 9 between sunrise 
and sunset and dropped again during the night. 

A similar tendency for biological agents to change the pH takes 
place in the ocean, but often the actual effect on pH is too small to 
be measured because of the high buffering capacity of sea water. In 
a study of the English Channel throughout a yearly period, during 
which photosynthesis varied from its maximum in the spring and 
summer to its minimum in the winter months, the pH of the upper 
water layer varied only from 8.16 to 8.25. 

The relative constancy of pH values in the sea is one more instance 
of the stability that characterizes the oceanic environment, and we 
may pause for a moment to throw a backward glance over the topic. 
We have noted the high degree of constancy in the open sea in re- 
gard to salinity, temperature, and oxygen supply, and now in relation 
to carbon dioxide, alkalinity, and pH. The ocean thus represents 
the largest natural environment in which highly stable conditions exist 
in respect to many ecological factors for weeks, months, or even years 



Ecological Effects of Carbon Dioxide 265 

together. R. E. Coker (1938) on the occasion of his retirement as 
president of the Ecological Society of America expressed this thought 
in the following way: 

It seems to have been the work of a Divine Providence to cover the 
greater part of the earth with the seas and at the same time to endow man- 
kind with no means of making any great change in them. "I will give you" 
he might have said, "dominion over the lands, over the beasts of the field 
and the birds of the air, and over the shrubs, trees and grasses, but over 
the oceans you shall have no control I trust you with small things, but the 
greater part of the surface of my beloved mundane sphere I shall keep 
under my own lock and key. By means of the seas, which you can not 
measurably modify, I shall proteqt you against your own follies." So the 
undisturbed high seas remain the great balance wheel of the terrestrial ma- 
chine, the chief source of the rainfall necessary for the continuance of or- 
ganic life on land, the real fountain head of the water whose return journey 
to the ocean carries the power required for our industries, the mainstay in 
regulation of temperature, the molder of climates. Concerning the signifi- 
cance of the seas to organic life anywhere, I do not need to say more to 
climatologists, geologists, or biologists. Suppose that man could have put 
his plow and machinery and his chemical reagents to work on the whole sur- 
face of the earth. Might he not long ago have decided that such a great 
expanse of brine was an error in Creation, as he has in effect concluded, 
and perhaps properly, with reference to the great areas of forests and grass- 
lands? Might he not have converted the saline waters into some other 
chemical form? Think how much more tasty or more useful the sea wa- 
ters might be were they turned into 20 per cent alcohol, for example, or 
into gasoline; and we might have had now to form a committee for the 
preservation of some small marine areas to be kept always as scientific "con- 
trols" and as refuges for the native population of non-alcohol addicted and 
non-gasoline tolerant diatoms, coccolithophores, Salpas, etc. very useless 
things in the eye of the practical man, but none the less interesting to the 
field biologist. 

But the designer of nature gave us no opportunity to bring such things to 
pass. "No," he might have said, "you may change the virgin grasslands 
into desert, you may pollute the waters and dam the streams as much as you 
please. You may accomplish much that is good by doing these things 
wisely. You mean well, but all the same, and for reasons that are quite sat- 
isfactory to myself, I put quite out of your reach the seas as the mainstay 
of organic life on your planet." And this is not as facetious as it may 
sound. 

Ecological Effects of Carbon Dioxide. The most fundamental 
effect of carbon dioxide in the aquatic environment is the part that it 
plays in the photosynthesis of green plants, just as is the case on land. 
We have seen that in most natural waters as in the atmosphere the 
amount of free carbon dioxide is very small. In sea water with a pR 
of 8.3, for example, less than 1 per cent of the total carbon dioxide 
is present in the free form (Fig. 7.9). However, both sea water and 



266 Oxygen and Carbon Dioxide 

hard fresh water contain a large reserve supply of carbon dioxide as 
bicarbonates and carbonates. Plants living in water of high pH must 
be able to get along by absorbing the tiny amount of free carbon 
dioxide present (which is then immediately replaced by dissociation) 
or else they must be able to use the carbon dioxide existing in com- 
bined form. Many pond plants have been shown to absorb the 
HCO : , ion and to use it in their photosynthesis (Steemann Nielsen, 
1952). Although we do not know whether most marine plants rely 
primarily on free or on combined carbon dioxide we find no evidence 
that the lack of carbon dioxide ever acts as a limiting factor for plant 
growth in the sea. 

In soft-water lakes, in which the total supply of carbon dioxide is 
small, extensive photosynthesis may reduce the concentration of this 
material to such an extent that further growth of the plants is pre- 
vented. Water is able to absorb carbon dioxide from the atmosphere 
only at a slow rate; the supply from ground water and from respira- 
tion and decomposition may be inadequate to meet the demand of 
the green plants. The withdrawal of carbon dioxide from the water 
also causes a concomitant rise in pH. If the change in hydrogen ion 
concentration is considerable, difficulty is frequently encountered in 
ascertaining to what extent the limitation of photosynthesis is due to 
lack of carbon dioxide and to what extent to unfavorable pH values. 
The growth of populations of planktouic algae in chemically fertilized 
fish ponds is believed sometimes to be curtailed by the exhaustion of 
the available carbon dioxide. Lime is commonly added to correct 
this situation. Improved growth of rooted vegetation in other soft- 
water ponds has been produced as a result of liming. 

The abundance of carbon dioxide also exerts certain specific effects 
upon the animals of the aquatic environment. The rates of some 
developmental and metabolic processes are increased at higher con- 
centrations of carbon dioxide and are decreased at lower values, but 
other animal reactions are inhibited by high carbon dioxide concen- 
trations, so that no general statement of its effect can be made. In 
vertebrates, and in some arthropods and mollusks the rate of respira- 
tory movements is definitely raised by increase in carbon dioxide 
tension, but in others this factor has little or no effect (Sheer, 1948). 

The concentration of carbon dioxide in the water influences its 
equilibrium in the blood of aquatic animals. An increase of the 
carbon dioxide in the blood causes a decrease in the oxygen affinity 
of vertebrate hemoglobins and of some invertebrate pigments (e.g., 
hemocyanin). In aquatic habitats with high carbon dioxide tensions 
animals with the type of blood strongly affected by carbon dioxide 



Ecological Effects of Carbon Dioxide 267 

in this manner would accordingly experience difficulty in obtaining 
oxygen. Fish inhabiting tropical swamps with water heavily charged 
with carbon dioxide have been found to possess a specially adapted 
type of blood that is little affected by the environmental carbon diox- 
ide. Species of fish dwelling in littoral areas where concentrations 
of carbon dioxide are above normal have similar adaptations en- 
abling them to extract oxygen from water of wider carbon dioxide 
range than is possible for pelagic fish. 

Changes in carbon dioxide also tend to modify the pH and the 
alkalinity of the blood. Alterations of these characteristics can be 
compensated for by changes in the abundance of hemoglobin and of 
cations such as Na and Ca, But these modifications of the blood 
equilibria require time. Fish that can tolerate but little variation in 
pH and alkalinity of the blood might well be seriously harmed by 
swimming rapidly across a sharp gradient, as at the thermocline, from 
water of low carbon dioxide to water of high carbon dioxide or vice 
versa. 

Another influence of carbon dioxide as an environmental factor is 
in relation to orientation. The movement of certain aquatic animals 
is affected by differences in concentrations of this gas. Reactions to 
increased carbon dioxide may have arisen in relation to the harmful 
effects of an excessive concentration of this gas mentioned above or 
in relation to the low oxygen tension commonly associated with it. 
A search has long been made for the factor or factors that guide 
anadromous fish migrating upstream to a specific tributary within 
which their spawning beds are located, and differences in carbon 
dioxide tension have been suggested as a possible orienting feature 
for salmon. Experiments were conducted by Collins (1952) on the 
alewife (Pomolobus), another anadromous fish, by dividing the 
stream, in which the fish were migrating, into two channels and add- 
ing carbon dioxide gas to the water of one channel. These tests 
demonstrated that the alewife can distinguish between small dif- 
ferences in free carbon dioxide content (down to 0.3 ppm) and that 
the majority (72 per cent) "chose" the stream with the lower amount 
of carbon dioxide. Supplementary tests showed that the fish were 
oriented primarily by the difference in carbon dioxide itself rather 
than by the concomitant change in pH. Whether a reaction of this 
sort to naturally occurring differences in carbon dioxide is a chief 
factor in directing migrating fish into one branch of a stream rather 
than into another remains to be investigated. Differences in tem- 
perature and other factors have been shown to be influential in the 
foregoing and in other observations, and evidence has been presented 



268 Oxygen and Carbon Dioxide 

by Hasler and Wisby (1951) and Hasler (1954) that olfactory re- 
sponses involving odor memory may play a part in the orientation of 
fish. 

Hydrogen Ion Concentration 

To turn to a more specific consideration of the ecological influence 
of pH in the free natural waters and in the interstitial water of muds 
and soils, the hydrogen ion concentration is affected not only by the 
reactions of carbon dioxide discussed above but also by other solutes 
present, both organic and inorganic. Since any alteration of the pH 
of natural waters is accompanied by changes in other physicochem- 
ical aspects of the medium, the ecologist must constantly be on guard 
against assuming that the easily measured pH exerts the controlling 
influence before determining what effect related changes in the 
equilibria of the water may have. Edmondson (1945), for example, 
showed that certain sessile Rotatoria are very likely excluded from 
lakes by high bicarbonate concentrations, but not necessarily by high 
pH. 

Many early investigators believed that pH would prove to be an 
ecological factor of major importance in controlling the activities and 
distribution of aquatic plants and animals. Some later workers, go- 
ing to the opposite extreme, suggested that the pH of the environment 
has little or no importance. Present information indicates that the 
effect of pH as a factor is real, but limited and highly variable in its 
influence from group to group. As would be expected, pH is gen- 
erally of minor significance in the ocean because of its relatively con- 
stant value in the highly buffered sea water. 

In bodies of fresh water the usual hydrogen ion concentrations en- 
countered toward the middle of the pH scale seem to have little dif- 
ferential effect on the majority of the inhabitants. The distribution 
of many aquatic organisms is unrelated to pH over a wide range, and, 
when tested in the laboratory, these species often exhibit a tolerance 
to pH values well above and below those occurring in their natural 
habitats. Speckled trout (Salvelinus fontinalis) are found naturally 
in waters ranging from pH 4.1 to 8.5 and were subjected to values 
as low as pH 3.3 and as high as 10.7 without apparent harm. The 
rotifer, Monostyla bulla, is reported to tolerate a range from pH 3.7 
to 9.1; the water moss Fontinalis dalecarlica can withstand pH values 
from 3.0 to 10.5. On the other hand, some plants and animals thrive 
best under acid, others only under alkaline conditions, and others 
seem to require a nearly neutral medium. A species of Euglena is 
found in water of pH 1.8 draining from mines, and certain bacteria 



Hydrogen Ion Concentration 269 

and fungi tolerate values as low as pH 1.4. Other species of animals 
and plants live in highly alkaline waters: Lake Elmenteita in the 
African Rift with a pH of 10.7 to 11.2 supports a considerable fauna 
and flora (Jenkin, 1936), and Soap Lake, Washington, with a pH 
running up to 10 is found to contain a tremendous population of 
rotifers and claclocerans during the summer. In contrast to these 
examples of wide toleration, are instances of organisms that will live 
only within a very narrow range of hydrogen ion concentration, 
sometimes extending over less than a pH unit. The ciliated pro- 
tozoan Stentor coeruleus is reported limited to pH 7.7 to 8.0. These 
aspects of pH and other physicochemical factors of the aquatic en- 
vironment are reviewed more extensively by Allee et al. (1949, Ch. 
11), and by Welch (1952, Ch. 7). 

The pH of mud and of soils is frequently one of the most important 
characteristics of these substrata. A good general account of the 
ecological aspects of this highly complex subject will be found in 
Daubenmire (1947, Ch. 2), and more detailed treatments are avail- 
able in ZoBell (1946) and Waksman (1932). As we have seen, the 
pH of these aquatic and terrestrial substrata varies widely, depending 
on the nature of the parent material, the degree of weathering, and 
the extent of biological activity, including decomposition. Organisms 
inhabiting mud must be capable of withstanding pH values that may 
be considerably different from those of the overlying water. The 
floras of strongly acid soils, such as those developed from granitic 
rocks in the cold temperate regions, are characteristically different 
from the floras of alkaline limestone soils. Fungi are the chief or- 
ganisms of decay in acid forest soils, whereas acid-sensitive bacteria 
and earthworms are abundant only in soils with a more nearly neutral 
reaction. The striking difference even in the optimum pH range of 
plants is illustrated by the accompanying examples (Spurway, 1941). 



Bog rosemary (Andromeda glaucophylla) 3 . 0-5 . 

Blueberry (Vaccinium corymbosum) 4 0-5.0 

Wake robin ( Trillium erectum) 7.0-7.5 

Spleenwort (Axplenium parvulum) 7.0-8.5 

Stinking chara (Chara vulgaris) 7 . 5-8 . 5 

Plants frequently modify the pH of their substratum by the action of 
their root secretions and their decomposition products. This activity 
accounts for the occasional occurrence in limestone regions of species 
characteristic of acid soils. In such instances the pioneer plants have 
probably established themselves in microhabitats of lower pH, and 



270 Oxygen and Carbon Dioxide 

subsequent generations have gradually reduced the pH in the ad- 
jacent area. Thus we see that in mud and soils, as well as in the free 
water of the aquatic environment, the pH may act as a specific limit- 
ing factor for some species, but that it is most generally useful as an 
index of the overall conditions. 

Calcium Carbonate 

Another way in which carbon dioxide exerts an effect upon environ- 
mental conditions is through its influence on the formation and dis- 
solution of calcium carbonate ( lime ) . Carbon dioxide in water forms 
carbonic acid which dissociates to form bicarbonate and carbonate 
ions, and the latter reacts with calcium ions to form CaCO 3 : 



C0 2 + H 2 ^ H 2 C0 3 ^ H+ + HCOr ^ H+ + COj 
CO 8 - + Ca++ ^ CaCOa (ppt) 



The amount of these ions that will remain in solution is greater at 
higher hydrogen ion concentrations (lower pH) and, unlike most 
salts, at lower temperatures. Since a change in amount of carbon 
dioxide affects the equilibrium between the carbonate and bicarbonate 
ions, it also influences the equilibrium between the precipitated 
CaCO 3 and the Ca++ and CO 3 = ions in solution. If CO 2 is added ( as 
by respiration), more H 2 CO 3 is produced, and this will dissociate to 
form hydrogen ions, lowering the pH and reacting with the CO^ ions 
to form more HCO 3 ~ ions. In nearly neutral solutions the equilibrium 
is favorable to this reaction (Fig. 7.9). The reverse situation occurs 
when CO 2 is withdrawn (as by photosynthesis), with the result that 
more CaCO 3 is formed. In this way lime is often caused to deposit 
on or in the tissues of aquatic plants. 

As mentioned in Chapter 3, fundamental differences in the nature 
of soils depend in part upon the abundance of CaCO 3 . The amount 
of lime present is a result of the nature of the parent rock material as 
modified by the combined action of climatic and biological agents. 
In the pedocal soil-group division the ecological conditions are such 
that CaCO 3 tends to be retained in the upper soil horizons but in the 
pedalfer soil-group division carbonic acid and other acids, produced 
by the metabolism of roots and of soil microorganisms and as by- 
products of decomposition, tend to bring about the dissolution of any 
CaCO 3 present and thus to allow it to be leached away. Ca++ and 
other cations in the soil tend to be replaced by hydrogen ions, and 
this causes the soil to become progressively more acid. As soil acidity 
increases, ion replacement is accelerated and often results in a serious 



Calcium Carbonate 271 

loss of cations needed as plant nutrients, as will be considered further 
in the next chapter. Carbon dioxide is thus a vital part of a com- 
plex web of physicochemical interdependencies in the soil involving 
carbonic acid, pH, Ca++ ions, and CaCO 3 , each of which has significant 
ecological relations with the plants and animals living in the soil. 

Deposits of CaCO 3 in fresh water are referred to as marl, although 
the term is also sometimes used for lime deposits in other situations. 
Marl in fresh water is sometimes the result of the accumulation of 
shells, but more often it is produced by the activity of plants. In 
hard-water lakes which contain large amounts of Ca ++ ion, the removal 
of carbon dioxide by photosynthesis readily causes the precipitation 
of CaCO 3 usually deposited on or in the tissues of the plants them- 
selves. The pond weed Potamogeton often feels gritty because of the 
lime present, and Chara was named "stonewort" for the same reason. 
As much as 30 per cent of the dry weight of the latter plant is ac- 
counted for by CaCO 3 in its tissues. Certain bacteria and various 
types of algae, especially the Myxophyceae, also cause the formation 
of marl on the bottom of lakes and ponds. The alga Cladophora 
forms a biscuit-shaped deposit of lime about its tissues, and these 
"Cladophora balls," commonly attaining diameters of more than 8 
cm, may accumulate like paving stones over the bottom or along the 
shore. 

In soft-water lakes and particularly those with an acid reaction, 
we find the reverse situation. Ca ++ and CO-f ions are not only less 
abundant but they also tend to stay in solution, with the result in 
extreme instances that the deposition of CaCO 3 by the activity of 
organisms is difficult or impossible. The Ca ++ ion plays an essential 
role in membrane permeability and other physiological processes of 
plants and animals. CaCO 3 is a principal component of the shells 
of mollusks and worms, and is incorporated in the exoskeletons of 
arthropods where it serves to add stiffness. The skeletons of verte- 
brates are composed of about 80 per cent calcium phosphate and a 
considerable portion of calcium carbonate and other calcium com- 
pounds. It is not surprising, then, to find that some animals are 
limited by subminimal supplies of Ca ++ and CO 3 " ions in the water. 
Robertson (1941) reports that Gammarus pulex requires at least 5 
mg Ca per liter before it can harden its exoskeleton. As a general 
rule few mollusks are found in acid lakes; those occurring are specially 
adapted to withstand the unfavorable conditions and characteristically 
display thin shells, often protected by a covering of chitin. How- 
ever, the bivalves Pisidium and Campelorna were recorded by Morri- 
son ( 1932) in water of pH 5.1 to 6.1 containing 3 to 5 mg Ca per liter 



272 Oxygen and Carbon Dioxide 

with shells nearly as heavy as in most alkaline waters. These animals 
were also found in water of pH 5.1 with as little as 1 mg Ca per liter, 
but the shells were poorly calcified. 

Since the ocean tends to be relatively constant in regard to many 
of its ecological factors, we might expect to find little variation in 
the conditions of lime formation in the marine environment. Ac- 
tually, however, sea water is so nearly saturated with Ca** and CO 3 = 
ions that slight changes in temperature, pH, and carbon dioxide are 
sufficient to throw the equilibrium just above or just below the satura- 
tion point. The colder and less alkaline parts of the sea are under- 
saturated in respect to Ca** and CO 3 ", and the water can take up ad- 
ditional amounts; the warmer regions of the ocean are supersaturated 
and CaCO 3 is readily formed, chiefly through the activity of various 
biological agents. The circulation of water in the great ocean basins 
is thus accompanied by a cycle in which Ca 1 * and CO 3 = ions tend 
to be taken up in polar or deep water, and lime deposits tend to ac- 
cumulate in shallow areas of the tropical seas (Fig. 7.10). As surface 

POLAR rCst 4 and CoJ from rivers TROPICAL 



Cold * 


pH 8. 1-8.3 /Coral \ Warm 


r~4 .. . 


_ * reefs * ^ 




' Supersaturation 

T i 


1 


CaC0 3 deposits f 
in shallow seas / 


1 


''.:;;::::::!.'' / 


Ca^andCOj 


/ 


f in solution 




I 


/ 


Undersaturation 


^ ^ . 


Cold 


pH7.5-7.9 Cold 



FIG. 7.10. Schematic longitudinal section of northern half of Atlantic Ocean, in- 
dicating the cycle of CaCO 3 . The arrows indicate the north-south and vertical 
components of the oceanic circulation. 



water moves toward the poles it becomes undersaturated in spite of 
the new quantities of Ca + + and C(V added by run-off and river dis- 
charge, and the deep return flow remains undersaturated because of 
the prevailing low temperatures and low pH values. But, as the 
water rises toward the surface in the tropics, the increase in both tem- 
perature and pH bring about saturation and then Supersaturation. 



Calcium Carbonate 273 

These conditions favor the secretion of lime and the building of coral 
formations in the tropical oceans. 

Floating in the open water of the warmer seas are countless trillions 
of planktonic organisms that use the easily accessible Ca ++ and CO 3 = 
ions to build their shells. Prominent among these are biflagellated 
coccolithophorids, foraminiferans, and pteropods. When these or- 
ganisms die, their calcareous skeletons rain down upon the bottom, 
and, if the water is not too deep, their remains accumulate to take 
part in the formation of globigerina ooze and pteropod ooze. Globi- 
gerina ooze is particularly extensive in the Atlantic and Indian Oceans, 
and this type of bottom deposit sometimes contains as much as 86 
per cent CaCO 3 . In deeper regions of the ocean with lower tempera- 
tures and pH values the bottom deposits contain much less calcareous 
material. These areas of the sea bottom are covered with red clay, 
or have deposits of diatom ooze or radiolarian ooze formed from the 
remains of plankton with siliceous skeletons that strongly resist solu- 
tion (Kuenen, 1950, Ch. 5). 

Calcareous formations around shores and on shoals in the marine 
environment are the result chiefly of the activity of bottom-living or- 
ganisms. Calcareous sediment resulting from wave action is de- 
posited on the bottom and may possibly be supplemented in some 
instances by direct precipitation from supersaturated water. The 
plants and animals taking part in lime production range from micro- 
scopic forms to large solitary species and those that form huge colonial 
aggregations. Particularly notable are certain bacteria, specialized 
representatives of the Protozoa, Coelenterata, Annelida, and Crus- 
tacea, as well as the calcareous algae and a great many kinds of 
mollusks (Sverdrup et al, 1942, Ch. 20). 

Lime-forming organisms are present in all seas, but they are much 
more prominent in warm waters. Although the physiological proc- 
esses involved in shell secretion are highly complex (Bevelander, 
1952; Wilbur and Jodrey, 1952) and not thoroughly understood, we 
do know that shell formation is greatly accelerated at high tempera- 
tures. In really cold water shell development tends to be suppressed, 
and exoskeletons of many invertebrates are thin or even absent, as in 
the naked species of the pteropod Clione. Shells in tropical regions 
are typically larger and thicker than those of related species in tem- 
perate regions and often display extra spines, ridges, or other pro- 
tuberances. The grandfather of them all is Tridacna, a bivalve mol- 
lusk with a shell that may approach 2 m in length and weigh 250 kg 
(Fig. 7.11). These animals, hidden among the coral formations, are 
large enough to be a danger as a man trap since they may close on the 



274 Oxygen and Carbon Dioxide 

foot of a person walking on the reef or on the hand of a pearl diver 
in deeper water. 

Since higher temperatures facilitate the secretion of CaCO 3 and 
accelerate the growth of organisms producing calcareous structures, 
we are not surprised to find that coral reefs are most abundant in the 
western portions of the tropical oceans where extensive areas of warm 
water are found (Fig. 7.12). Coral animals themselves require a 
temperature above 20 C for good growth, and many other organisms 
taking part in reef formation flourish only in tropical seas. Foramini- 
ferans, millepores, alcyonarians, barnacles, serpulid worms, and mol- 




Photo J. I. Tracey, Jr., U. S. Geological Survey 

FIG. 7.11. Underwater photograph of the giant clam Tridacna on the reef flat at 
Bikini. The specimen is about 45 cm long. 

lusks growing in unbelievable profusion among the corals add their cal- 
careous parts to the formation. Certain green algae, such as Halimeda, 
often contribute large amounts of lime to the floor of the lagoon. The 
reef would not grow as an enduring structure, however, if it were not for 
the cementing action of lime-secreting Bryozoa and particularly of cer- 
tain red algae, such as Lithothamnion. These plants, known as encrust- 
ing corallines, or nullipores, often produce a sort of pavement protecting 
the new reef growth from destruction by the surf and thus contribute 
enormously to reef formation although they are not as conspicuous as 
the coral animals (Gardiner, 1931; Ladd and Tracey, 1949; Kuenen, 
1950, Ch. 6). The lavish development of calcareous structures (Fig. 



Calcium Carbonate 



275 




FIG. 7.12. World distribution of coral reefs (Ekman, 1953, after Schott) in rela- 
tion to the 20 C isotherm for the coldest month in the year. ( Hutchins and 
Scharff, 1947, in the /. of Marine Research. ) 




Photo R. W. Miner, Am. Museum of Nat. Hist. 



FIG. 7.13. View of coral structures at depth of 8 m on Andros Reef, Bahamas. 



276 Oxygen and Carbon Dioxide 

7.13) and the extraordinary variety and beauty of the reef community 
will never be forgotten by anyone who has walked on a reef at low 
tide, or, better, has explored one with a diving helmet. 

These spectacular coral formations are the result of equilibria in 
the circulation of Ca* + and CO 3 = ions that are dependent upon physical 
chemical, and biological agents. In the words of Redfield (1941): 
"The calcium cycle involves not only exchanges between land, sea, 
and the bottom. It includes the consideration of the metabolic cycle, 
the CO 2 cycle, the temperature cycle, and the cycle of movement of 
water between the surface and the depths. One could not find a 
better example of the necessity of considering the environment as a 
whole as a unified system." 

In the foregoing discussion of the ecological influences of oxygen 
and carbon dioxide we have observed that these "sister" materials, 
reciprocally involved in photosynthesis and respiration, exhibit a 
great contrast in the circumstances of their occurrence and in their 
variability. Oxygen is generously abundant in the atmosphere, 
whereas the chief reservoir of available carbon dioxide is in the ocean. 
We have found that in certain habitats either of these substances may 
be so reduced in amount by the physiological processes of the in- 
habitants as to curtail the life activities of the organisms themselves. 
We have also reviewed the particularly intriguing further complica- 
tions brought about by the fact that carbon dioxide reacts with other 
materials in the environment to influence the equilibria involving pH 
and CaCO 3 . All of these relations with oxygen and carbon dioxide 
are thus further striking demonstrations of the mutual interde- 
pendence of the organism and its environment. 



8 



Nutrients 



NUTRIENTS AND THE ENVIRONMENT 

A student once wrote in an ecology paper; "For all organisms nutri- 
tion is par excellence" his way of stating that nutrients are a crucially 
important ecological influence! Indeed, all living things are de- 
pendent upon the environment for the supply of energy and of 
the materials necessary for their nutrition. Green plants use sun- 
light as a source of energy and synthesize carbohydrates from water 
and carbon dioxide, obtained as discussed in earlier chapters. To 
produce living tissue other materials must also be presentparticu- 
larly protein. Plants build their own proteins from carbohydrates, 
nitrogen compounds, and other inorganic substances. These simple 
inorganic building materials constitute the nutrients of the green plant. 

Colorless plants and animals must have ready-made organic com- 
pounds for their nutrition, and they feed upon material which is, or 
recently has been, part of another living organism. Energy is ob- 
tained by the oxidation of this organic food. Nutrients for green 
plants are thus obtained from the supply of inorganic substances in 
the environment, whereas nutrients for animals and colorless plants 
are represented by the organic matter derived from other organisms. 

Nutrition is obviously and necessarily a reciprocal process. The 
availability of nutrients in the environment influences the growth and 
distribution of animals and plants, and the activities of living or- 
ganisms profoundly change the abundance of nutrient materials. 
The oxidative metabolism of the living organism and the oxidative de- 
composition of all plant and animal tissues after death result in the 
return to the environment not only of water and carbon dioxide but 
also of inorganic materials that can serve again as nutrients for green 
plants. These relationships are indicated in the following scheme 
which might be thought of as the "equation of all outdoors": 

277 



278 Nutrients 

Green Plants + O 2 

H 2 O + C0 2 + Nutrients + Energy / 

\Animals 

Colorless plants 

Modes of Nutrition 

The various modes of nutrition in the living world may be classified 
according to the accompanying scheme. 

Autotrophic 

Holophytic (Phototrophic) 

Chemotrophic 
Heterotrophic 

Holozoic 

Saprophytic 

Parasitic 
Mixotrophic 

Autotrophic, or "self-nourishing," organisms can synthesize all es- 
sential organic components entirely from inorganic substances; they 
are therefore not directly dependent upon other organisms for food. 
This group composed principally of holophtjtic forms which use light 
as a primary source of energy through photosynthesis is represented 
most prominently by the green plants, although the purple bacteria are 
also included. The chemotrophic organisms, lacking chlorophyll, are 
not able to make direct use of solar energy but instead derive their 
energy from the oxidation of certain inorganic substances. This spe- 
cialized mode of nutrition is limited to relatively few kinds of organ- 
isms. Among those encountered frequently in natural environments 
are the sulphur bacteria, which oxidize hydrogen sulphide first into 
free sulphur and then into sulphate compounds, and the iron bacteria, 
which oxidize ferrous salts to ferric salts. 

Heterotrophic organisms require already formed organic com- 
pounds as a basis for their nutrition and hence are dependent upon 
autotrophic organisms for food directly or indirectly. The most 
prominent subdivision of this group is composed of animals that dis- 
play the holozoic mode of nutrition. These free-living animals char- 
acteristically ingest solid food and digest it internally. Saprophytic 
organisms commonly lack a digestive cavity and absorb organic food 
directly from the environment, usually employing external digestion. 
Included in this category are most bacteria, many fungi, some flagel- 
lates, and a very few higher plants. The parasitic mode of nutrition 



Modes of Nutrition 279 

is utilized by some species in almost every taxonomic group of plants 
and animals. Parasites live on or in other living organisms and obtain 
organic food directly from the bodies of their hosts. 

Mixotrophic organisms are capable of both autotrophic and hetero- 
trophic modes of nutrition of one sort or another. Insectivorous 
plants possess special anatomical adaptations for entrapping insects 
and digesting them (holozoic nutrition) and at the same time carry 
on photosynthesis (holophytic nutrition). Many green flagellates 
similarly display the capability of utilizing either of these types of 
nutrition, and, since they are also motile, it is thus a constant source 
of discussion whether to classify these forms as plants or as animals. 
When light is available these flagellates can photosynthesize like 
typical green plants, but at other times they live saprophytically. 
Euglena, for example, is found living in this manner in the decompos- 
ing organic matter on the bottom of a pond. Other green flagellates, 
are provided with a functional mouth and are capable of the in- 
gestion of solid food particles like a typical animal. The green alga 
Chlorella is another Dr. Jekyll and Mr. Hyde, since it is capable of 
either phototrophic or saprophytic nutrition, and other members of 
the phytoplankton may be found to lead this double life. 

The fact that most animals and colorless plants require for their 
nutrition the organic matter of other organisms means that the food 
of these heterotrophic forms has an ecology of its own. The food 
organisms themselves grow, change, move around, and die. Further- 
more, many of these prey species not only serve as a source of nutri- 
ment for the predators but may also influence them in other ways as 
part of their environment. For animals, then, our consideration of 
nutrients as an ecological factor overlaps the general consideration 
of the presence of other organisms as environmental influences which 
will be taken up in subsequent chapters. 

But with green plants the situation is quite different. Here the 
nutritional needs are substances that are relatively simple in the 
chemical sense. The absorption of nutrients from the environment 
by plants is a relatively direct and much less complicated process than 
the food getting of animals. This fact is no doubt one reason why 
certain aspects of ecology were first developed by botanists. With 
the holophytic organisms we can often see more clearly the limitation 
of growth and distribution by the influence of nutrients. We shall 
accordingly consider first the ecological aspects of the nutritional 
requirements of green plants since these are by far the most important 
of the autotrophic forms. Then we shall discuss the significance of 
the nutrient factor for animals and other heterotrophic forms, includ- 



280 Nutrients 

ing the saprophytes involved in the "return" processes of decomposi- 
tion and regeneration. 



Influence of Nutrients on Green Plants 

Nutrients Required. The nutritional requirements of green plants 
embrace several major materials and a longer list of minor substances. 
The principal elements that go into the construction of plant tissues 
are: carbon, oxygen, hydrogen, and nitrogen. We have already dis- 
cussed the sources of carbon from CO 2 , of hydrogen from H 2 O, and 
of oxygen from O 2 , CO 2 , and FLO. An enormous supply of nitrogen 
exists in the air but atmospheric nitrogen is unavailable to most 
plants. In this respect nitrogen presents a complete contrast to CO 2 . 
We have seen that the quantity of CO 2 in the atmosphere is extremely 
small but that it is directly and readily available to plants. Since 
nitrogen makes up 79 per cent of the atmosphere, terrestrial plants 
would seem to be practically surrounded by this gas yet only a few 
special types of bacteria can use free nitrogen. Aquatic plants are 
similarly unable to utilize directly the large amount of nitrogen gas 
dissolved in natural waters. Green plants must obtain their nitrogen 
from nitrogen compounds; these are not generally plentiful and are 
derived primarily from organic decomposition. Most plants grow 
best when supplied with nitrate many can obtain their nitrogen only 
in that form but others such as some green algae can assimilate 
nitrite, ammonia, and amino acids (Algeus, 1951). 

Two other elements necessary in moderate amounts for the growth 
of holophytic plants are sulphur, which is abundant in the soil and in 
the sea as sulphate, and phosphorus, which occurs as phosphate but 
is not at all plentiful. Although some phosphate is derived from the 
disintegration of parent rock, the decomposition of organic matter is 
the immediate source of this material for most green plants. Essen- 
tial in smaller quantities are potassium, calcium, and magnesium. 
Potassium originates from potassiferous silicates found in almost all 
rocks; the occurrence of calcium has been discussed in the previous 
chapter. Magnesium, the keystone in the structure of chlorophyll, 
is usually sufficiently available in the soil and in natural waters. 

Other elements known to be necessary for the growth of plants at 
least in trace quantities include: iron, manganese, copper, zinc, 
molybdenum, and boron (Stiles, 1946). The fact that plant growth 
can be affected by zinc in the amount of one part in 200,000,000 
illustrates the minuteness of the quantities of these elements that may 
be effective. Silicon is used by diatoms in the construction of their 



Law of the Minimum 281 

shells; other elements such as aluminum, fluorine, and bromine are 
taken up by certain plants but seem to have no nutritional significance. 
Although sodium and chlorine are commonly present in plant tissues, 
these substances are probably not necessary for most species. Cer- 
tain elements found in plants and also in animals have been concen- 
trated hundreds or even thousands of times more than they are in 
the environment. This condition is strikingly true of trace elements 
and is easily apparent in the sea where the chemical composition of 
the surrounding water is highly uniform. Silicon and iron are greatly 
concentrated in the bodies of diatoms, and titanium, known to occur 
in certain marine organisms, has not yet been detected in free sea 
water (Sverdrup et al., 1942, Ch. 7). Vanadium may constitute al- 
most 0.2 per cent of the dry weight of Ascidia mentula. Since vana- 
dium is present in sea water in a concentration of only 0.3 to 0,6 mg 
per cu m, the element has been concentrated roughly four million 
times in the body of this animal. 

Several other elements, that may or may not be essential to the 
plants, are nevertheless significant for the animals feeding upon the 
plant material as being either beneficial, e.g., cobalt, iodine, and 
nickel, or harmful, e.g., selenium and molybdenum. Few if any of 
these minor nutrients exist as elements in soil or in the water; they 
mostly occur and must be absorbed as salts or ions in chemical com- 
bination with other elements. Above-normal amounts of some of the 
trace elements are definitely injurious to plants often with only a 
narrow range between minimal, optimal, and harmful concentrations. 
For example, an increase of the boron concentration in the soil to 
1 ppm will kill some plants, whereas for other species 1 ppm is optimal 
and 5 ppm are toxic, 

Law of the Minimum. If any necessary nutritive element is com- 
pletely lacking, the growth and eventually the maintenance of a plant 
will obviously be prevented. In some habitats all the essential sub- 
stances may be present, but one or more of them may exist in con- 
centrations so low that certain species cannot absorb them rapidly 
enough to satisfy their nutritional needs. Under these conditions 
the growth of the plants will be limited in conformity to Liebig's law 
of the minimum. This law states that growth is limited by the sub- 
stance that is present in minimal quantity in respect to the needs of 
the organism. Liebig's law was originally delineated, and is best 
applied, in relation to limitation by nutrients, but it is sometimes 
used in a broader sense to include limitation by other factors of the 
environment. 

The reader should understand that the limiting substance is the 



282 Nutrients 

nutrient that is minimal relative to requirements and not necessarily 
the one that occurs in the smallest absolute amount. In this sense 
the law of the minimum in ecology is analogous to the law of com- 
bining weights in chemistry. The following equation gives the pre- 
cise amounts of Na and Cl that will combine to form NaCl: 

23g Na + 35g Cl -> 58g NaCl 

If lOOOg of Na were added to 35g of Cl, no more than 58g of NaCl 
would be formed. Under these circumstances the 35g of Cl limits 
the total amount of NaCl produced. If, however, 23g of Na were 
allowed to combine with 34g of Cl, somewhat less than 58g of NaCl 
would be obtained. The 34g of Cl is now the limiting substanfce, 
although, in this instance, it is not the smaller quantity of the two 
components. 

The same sort of limitations occur in the growth of plants, but the 
amounts of nutrients absorbed are not nearly as rigidly fixed as the 
combining weights in simple inorganic reactions. Much has been 
learned from water culture methods regarding the variations in the 
quantities of nutrients used by higher plants in normal growth and 
also regarding minimal requirements (Hewitt, 1952). Sometimes 
the amounts or the ratios of nutrient materials used are completely 
altered at different seasons. For example, a certain group of roses 
tested in July, took up 450 ppm N and 250 ppm K, whereas in Decem- 
ber they absorbed 150 ppm N and 750 ppm K (Turner and Henry, 
1948). It has also been learned that a surplus of some nutrients 
can in certain instances make up for the lack of others another illus- 
tration of the principle of partial equivalence. Experiments have 
shown that Na is necessary for the growth of the sugar beet, regard- 
less of the amount of K that may be present, although Na is not 
essential for most plants. Furthermore, if sugar beets are grown 
with insufficient K, the symptoms of K deficiency are practically elim- 
inated by supplying extra Na (Lehr, 1942). 

Similar information on the variation in nutrient requirements has 
been obtained from cultures of algae. The amount of one nutrient 
taken in is sometimes found to depend upon the amounts of other 
nutrients present, and the ratio may change radically according to 
circumstances. Ketchum (1947) found that the rate of assimilation 
of phosphorus by the diatom Nitzschia closterium was related to the 
concentration of both phosphate and nitrate in the medium, whereas 
the uptake of nitrate was independent of the concentration of phos- 
phate and related only to the concentration of nitrate. Various indi- 
vidual cultures grown in the same medium varied by as much as 50 



Limitation by Nutrients in Nature 283 

per cent in their phosphorus content (Ketchum and Reclfield, 1949). 
Some cultures multiplied threefold in deficient media and contained 
only y 5 the normal amount of phosphorus. These algae not only 
could make up their deficiency in one day when again supplied with 
phosphate, but also would absorb more of this material than neces- 
sary if excess phosphate were available. 

Although a certain degree of flexibility exists in the use of nutri- 
ents by plants, in their ability to produce some growth under deficien- 
cies, and in the possible partial substitution of one nutrient for an- 
other, an ultimate minimum is eventually reached for the availability 
of the required elements. If the plants or the nutrients are mobile, 
the lack of nutriment will affect the whole population at about the 
same time. 

Urban sewage and certain other pollutants discharged into rivers 
and lakes provide extra nutrients that frequently cause the growth 
of excessive quantities of algae that are unsightly and odoriferous. 
Practical application of the law of the minimum has been suggested 
in controlling this objectionable growth. Since the treatment of all 
the sewage effluents is costly, consideration has been given to the 
removal of one essential nutrient only. After the elimination of phos- 
phate from the sewage in pilot tests in Wisconsin the growth of algae 
was greatly curtailed, although all other nutrients were present in 
great abundance. The feasibility of using this procedure on a large 
scale is being investigated. 

Pollution from duck farms bordering Great South Bay, New York, 
cause the water of the bay to be rich in reduced nitrogen compounds 
and to furnish a low ratio of nitrogen to phosphorus. This unusual 
situation in regard to the nutrient factor, in combination with other 
special environmental conditions, has resulted in the suppression of 
the usual type of phytoplankton in Great South Bay and the produc- 
tion of tremendous quantities of small green algae (Ryther, 1954a). 
This alteration in the phytoplankton population appears to have pre- 
vented the oysters in the bay from feeding and growing normally, 
with consequent serious reduction in the oyster fishery. 

Limitation by Nutrients in Nature. In the terrestrial environment 
nutrients exert a control over the growth and distribution of plants 
primarily as a result of deficiencies, although excessive quantities of 
harmful substances occasionally exclude plants from particular areas. 
Availability of suitable nutrients is, of course, only one aspect of the 
soil that determines whether a given species can grow successfully; 
physical texture, moisture, pH, and other chemical aspects of the 
environment, as well as the climate, must also be suitable. Emphasis 



284 Nutrients 

should again be placed on the fact that these factors are often related 
and their effects mutually interdependent to a considerable extent. 

The most commonly deficient nutrients in the soil are phosphate, 
nitrate, and potassium. A general discussion of the factors tending 
to conserve or to deplete these materials in the terrestrial substratum 
was presented in Chapter 3. The existing concentration of these 
nutrients depends upon the composition of the parent rock material 
and the extent of the modification of it by leaching and upon the 
decomposition of biological products. Our information on the limita- 
tion of plant growth by lack of phosphate, nitrate, or potassium in the 
soil is derived chiefly from agricultural research. A consideration 
of this vast body of knowledge is obviously beyond our present scope, 
but the reader will find further discussions in the Year Books of the 
U.S. Dept. of Agriculture, in Meyer and Anderson (1952), and in 
Lyon, Buckman, and Brady (1952). 

Under natural conditions the phosphate removed from the soil by 
plant growth is in part replaced by the breakdown of the parent rock, 
although this process is extremely slow and some rocks contain little 
or no phosphorus. Of much greater significance in the restoration of 
phosphate to most soils is the decomposition of organic matter. Since 
this material is derived from the bodies of dead organisms and from 
animal excreta (Hutchinson, 1950), the process represents a critical 
instance of the reciprocal action of organisms on their environment. 

Replenishment of nitrate and of potassium in the soil is similarly 
dependent primarily on the decomposition of organic material, but 
the latter element is also derived from potassiferous silicates found in 
most rocks. Supplementary supplies of nitrate are derived from am- 
monia formed in rain water by electric discharges in the upper air 
(Hutchinson, 1944) and from the fixation of free nitrogen by certain 
soil bacteria. 

Not only must the essential nutrients occur in the soil in sufficient 
quantity, but, even more important, they must be present in available 
form. Iron, manganese, magnesium, zinc, and sometimes phos- 
phorus remain in essentially unavailable states in soils that are too 
alkaline. On the other hand, under strongly acid conditions phos- 
phorus forms insoluble phosphates with iron and aluminum, nitrates 
cannot be readily formed from ammonia, and certain elements may 
become so soluble as to attain toxic concentrations. One of the chief 
reasons for adding lime to agricultural soils is to correct unfavorable 
acidity and thus to render nutrients available for absorption by the 
plants. 

Calcium has important effects as a nutrient in addition to the part 



Limitation by Nutrients in Nature 285 

it plays in influencing pH and other aspects of the environment. Al- 
though some plants do not germinate if more than a trace of calcium 
is present, many more species are dependent upon a generous supply 
of this element. Plants such as Aster amellus and Libanotis montana 
that require a high percentage of calcium in the soil are referred to as 
obligate calciphytes or calciphiles, and their occurrence is limited to 
limestone or dolomitic regions. Many species are facultative calci- 
phytes. Other plants such as Calluna (heather) and Vaccinium 
(blueberry) that occur where the soil contains less than 3 or 4 per 
cent calcium, are known as calciphobes. Investigation shows that 
these correlations may be primarily due to the physical structure of 
the soil, its general richness, or its pH as indirectly influenced by the 
presence of lime, rather than to a direct nutritive dependence upon 
calcium. The occurrence of Sphagnum, formerly always mentioned 
as a strong calciphobe, is now believed to be adversely affected by high 
concentration of hydroxyl ions rather than of calcium. This compli- 
cated subject is discussed more fully by Lundegardh (1931, Ch. 7). 

Soils containing large amounts of gypsum (hydrated calcium sul- 
phate) or of serpentine (hydrated magnesium silicate) support 
peculiar or impoverished floras. Some kinds of plants require gyp- 
sum but others are intolerant of it. Johnston ( 1941 ) reported that in 
the deserts of northern Mexico the complex pattern of distribution in 
certain plants was controlled with remarkable rigidity by their de- 
pendence on gypsum coupled with very successful powers of dis- 
semination. Haploesthes Gregii (a grass) and Namn Stcwartii, for 
example, were often found in widely separated patches of gypsum 
soils no more than 2 or 3 sq m in extent. Within the same genus one 
species, Fouquieria shrevei, was confined to gypsum areas whereas 
another species, F. splendens, occurred only on non-gypsum soils. 

Serpentine can be tolerated by only a restricted group of plant 
species, and the areas of sparse vegetation growing on soils rich in 
this material are known as "serpentine barrens." South of Phila- 
delphia barrens of this type may be seen as areas about half a mile 
wide and several miles long in which pitch pine, black-jack oak, 
scrub oak, and cat briars are the most prominent species, whereas 
surrounding regions support good stands of large red and white 
oaks and gums with few if any pines (Wherry, 1932). A Venezuelan 
orchid, Epidendrum O'Brienanum, exhibits a similarly interesting as- 
sociation with iron ore deposits; the plant grows within areas con- 
taining this ore but is not found beyond their peripheries (Buck, 
1949). 

In other localities deficiencies in trace elements control plant 



286 Nutrients 

growth. In certain parts of Australia hardly any production of clover 
could be obtained although general growth conditions and the supply 
of common nutrients in the soil seemed to be adequate. After a long 
study it was discovered that required trace amounts of molybdenum 
were lacking in these areas. When the astonishingly small quantity 
of 5g of molybdenum per hectare ( % 6 oz per acre ) was added to 
the soil, a thick growth of clover more than 30 cm high was obtained. 

As a result of generations of study of the nutrient needs of agricul- 
tural plants we have accumulated a considerable knowledge regarding 
the use of commercial fertilizers for our common annual crop plants. 
Bags of chemical fertilizers that we see being applied to farms bear 
designations such as 6-8-4 or 5-10-5. These numbers refer to the per- 
centages of total nitrogen, available phosphoric acid (P 2 O 5 ), and 
water-soluble potash (K 2 O) that are contained in the fertilizer. 
Great success has been attained in supplying these commonly de- 
ficient materials to agricultural lands by means of such chemical fer- 
tilizers. However, we still know extremely little about deficiencies in 
trace elements and in the more complicated organic compounds. 
These materials also are leached from the soil by rain, and removed 
from the area when crops are harvested. In the movement known 
as "organic farming" the plowing-in of green manure and other types 
of organic materials is especially recommended with the hope of re- 
plenishing as many as possible of the trace elements and organic 
constituents that are removed from the soil as the result of farming. 
Crops grown with a full supply of all nutrients, minor and major, will 
be more vigorous, more resistant to disease and provide more com- 
plete nourishment for man and the domestic animals using them 
as a source of food. 

We have much less information about the needs and deficiencies 
of nutrients for natural vegetation in uncultivated areas. For forest 
land the time scale is entirely different, and the critical nutrients may 
also be different. When a crop of trees is harvested after 40 years' 
growth, we do not know what loss of nutrients the soil has sustained 
during this long period, nor do we know how to treat forest land to 
maintain its productive capacity. 

Aquatic plants have the same general nutrient requirements as ter- 
restrial plants, and, interestingly enough, two of the elements that 
are critical on land, nitrogen and phosphorus, are also likely to be 
seriously scarce in the water environment. Potassium is ordinarily 
sufficiently abundant in fresh water, and reference to Table 3 will 
indicate that it is one of the six most abundant ions in sea water. 
Other materials occurring in small or trace quantities may sometimes 



Limitation by Nutrients in Nature 287 

act as limiting factors for the growth of aquatic plants. The availa- 
bility of nutrients in fresh water and in the sea is complicated by the 
following facts: (1) the supply and the availability of these materials 
are involved with circulation and various chemical equilibria in the 
water, (2) the amounts of the nutrients needed by the plants vary 
according to their physiological condition, and (3) the growth of the 
plants significantly depletes the supply. For a more detailed discus- 
sion of these complicated interrelations than is possible here the reader 
should refer to such treatments as those of Ruttner (1953) or Sverdrup 
et al. (1942, Ch. 16). 

As would be expected, the amounts of nutrient salts in fresh water 
vary widely from lake to lake and also may change greatly within 
the same body of water from season to season, as rates of plant growth 
and organic decomposition wax and wane. Differences in availabil- 
ity of plant nutrients in the water is the principal criterion upon which 
European limnologists have divided lakes into three major types: 
oligotrophic, eutrophic, and dystrophic. Although difficulty exists in 
applying this classification universally (Welch, 1952), the following 
general distinctions can be made. Oligotrophic lakes, typically very 
deep, are poor in phosphorus, nitrogen, and calcium; electrolytes and 
organic materials are low, but oxygen is abundant at all depths and 
in all seasons. Eutrophic lakes are relatively shallow, and are typi- 
cally rich in plant nutrients and in organic materials; electrolytes are 
variable in abundance; oxygen is depleted seasonally and may be en- 
tirely absent in the hypolimnion. Dystrophic lakes, occurring prin- 
cipally in bog surroundings or old mountains, are abundantly supplied 
with phosphorus, nitrogen, calcium, and organic materials, but the 
growth of most lake organisms is limited by the occurrence of high 
concentrations of humic substances. Electrolytes are low, and 
oxygen is almost or entirely absent in deeper water. 

In view of the stress laid upon the constancy of the ocean as an 
environment in many respects including its salinity, it might be sup- 
posed that all nutrient salts in sea water are uniformly abundant. 
Such is far from the truth, however. The salinity of the ocean is de- 
termined almost entirely by the abundance of the salts listed in Table 
3, and these are measured in parts per thousand. Phosphate and 
nitrate are not found in this table at allthey occur in quantities of 
the order of parts per billion. The paucity of these nutrients is known 
to be frequently responsible for the curtailment of plant growth in 
the sea. 

Other materials occurring in small or trace quantities may act as 
limiting factors in the sea. Silicate has been reported upon occa- 



288 Nutrients 

sion as insufficient for the production of the siliceous shells of diatoms. 
The importance of manganese for the growth of phytoplankton is 
discussed by Harvey (1949). In general, however, we know alto- 
gether too little about the occurrence of trace elements in natural 
waters in relation to the amounts needed for the healthy growth of 
aquatic organisms. Since artificial sea water prepared from the 
list of common constituents will riot support life indefinitely, we re- 
alize that other materials in minor quantities are essential for con- 
tinued growth. 

In both the fresh-water and marine environments particular in- 
terest has centered on the availability of phosphate and nitrate not 
only because these nutrients commonly become critically scarce but 
also because their depletion is often brought about by the growth 
of the plant population itself. We have little precise information on 
the rate of use of nutrients by phytoplankton under natural condi- 
tions or their minimum requirements, but a certain amount of data is 
available from laboratory experiments. In one carefully controlled 
set of tests a population of Chlorella vulgaris of 70 million cells per 
liter reduced the phosphorus content of its culture medium from 1.6 
microgram-atoms (/Ag-atoms) per liter to 0.4 /xg-atom per liter and in 
another test from 0.8 /Ag-atorn per liter to zero in 24 hours. The 
diatom Nitzschia frustndum in a population of 40 million cells per 
liter reduced its supply of phosphorus from 1.6 /xg-atoms per liter to 
zero in a similar length of time (Rice, 1949). The growth rate of 
these fresh-water species was sharply reduced at the lower concentra- 
tions of phosphorus. The multiplication of the marine diatom 
Nitzschia closterium was found to be independent of phosphorus 
concentration at values above 0.5 /xg-atom per liter, but below 0.3 
and especially below 0.15 ^g-atom per liter division rate was dras- 
tically curtailed. By summer the concentration of phosphorus in 
many natural waters has been reduced to critically low values for 
example to less than 0.1 in the English Channel and to less than 0.25 
/xg-atom per liter in western Lake Erie. 

A classical illustration of the seasonal exhaustion of nutrients by 
the marine phytoplankton is shown in Fig. 8.1 that represents the 
seasonal changes in phosphate and nitrate in the English Channel. 
The considerable supply of these nutrients present during the winter 
is rapidly reduced each spring by the growth of the phytoplankton 
sometimes to indeterminably small quantities. The concentration of 
nitrate and phosphate commonly remains very low during the sum- 
mer when no effective stirring of the water takes place. Meanwhile 



Limitation by Nutrients in Nature 289 

the growth of the diatoms is curtailed by the very depletion the 
diatoms have caused. Further growth cannot take place until the 
autumnal breakdown of thermal stratification allows a new supply of 
nutrients to be brought up from the deeper water layers by vertical 
turbulence. Sometimes a second period of diatom growth occurs 
during the autumn, temporarily reducing the concentration of nutri- 
ents again. Eventually reduced light and the more effective stirring 
caused by lower temperatures and stronger winds bring this autumnal 
growth to an end, and allow the surface layers to regain the higher 
concentration of nutrients characteristic of the winter period. 



I I I ! II I I I I u f I I I I I I 




1925 1926 

FIG. 8.1. Seasonal changes in availability of nutrients for marine phytoplankton as 

represented by annual cycle of phosphate and nitrate in the English Channel. 

( Harvey, 1928, Copyright, Cambridge Univ. Press. ) 

In bodies of water that are suitable in regard to size and drainage, 
such as many inland ponds, the seasonal depletion of nutrients may 
be prevented artificially by adding fertilizer to the water and thus 
removing this limiting factor for the growth of the plants. Organic 



290 



Nutrients 



manures have been supplied to fish ponds in the Old World for cen- 
turies, but in the 1930s the use of chemical fertilizers was begun on 
an expanding scale in the United States. By broadcasting fertilizer, 
such as "6-9-2" to "12-9-2," over the pond from the shore (Fig. 8.2), 
or from a boat, the concentration of nutrients is maintained continu- 
ously at such a level that phytoplankton can flourish through the warm 




FIG. 8.2. Broadcasting commercial chemical fertilizer over a pond in Belmont, 
Mass., to supply nutrients to the phytoplankton at the beginning of the food chain. 

season of the year. The thick growth of planktonic plants provides 
abundant food for copepods, cladocerans, and other types of zoo- 
plankton and for the bottom fauna, among which chironomids are 
particularly prominent, The resulting increase in the abundance 
of these invertebrates stimulates the growth of fish such as the blue- 
gill sunfish that feed upon them, and the enlarged population of 
these "forage" fish furnishes a rich food supply for predatory species 
such as the bass. In this way the production of pan fish or sport fish 
in the pond is significantly improved (Edminster, 1947; Meehean, 
1952 and succeeding articles in the Symposium on Farm Fish Ponds 
and Management). Similar augmentation in the growth of marine 
fish by the use of commercial fertilizer to stimulate the development 
of phytoplankton and subsequent links in the food chain has been 
demonstrated in Scottish sea-lochs (Gross, 1947; Raymont, 1950). 



Limitation by Nutrients in Nature 291 

In fertilized Loch Craiglin plaice accomplished 2 years' normal 
growth in 1 year (Fig. 8.3), and flounders accomplished 5 to 6 years' 
growth in less than 2 years. 

Growth of phytoplankton resulting from the fertilization of ponds 
may produce further effects of ecological importance. Dense 
"blooms" automatically cause the disappearance of benthic plants by 




Apr. June Aug. Oct. Dec. Feb. Apr. June Aug. Oct. Dec. Feb. Apr. 

Age 1 year 2 years 

FIG. 8.3. Diagram showing the greater growth of the flounder, Pleuronectes flesus, 

in Loch Craiglin (upper curve) after fertilizer was added compared with normal 

growth in an unfertilized loch (lower curve). (Data from Gross, 1947.) 

cutting off the light from the bottom. Bottom vegetation is generally 
undesirable in farm ponds that contain both prey and predator species 
of fish because its presence allows too many small fish to escape 
capture by the large fish. On the other hand, the growth of the 
planktonic algae must not be allowed to proceed to such a point dur- 
ing the summer that the decomposition of the algal material after the 
growing season will exhaust the oxygen supply and kill the fish during 
the winter. For this reason the addition of fertilizers must be cur- 
tailed in northern regions where the winter is long. Carefully con- 
trolled experiments carried out in relation to practical fish farming 
have provided valuable quantitative data on nutritional and other 
ecological relationships involved in the aquatic environment. At 
the same time the failure of hasty or ill-conceived fish farm practices 
form an admirable illustration of the necessity for a thorough under- 
standing of ecological principles as a background for successful con- 
servation or cultivation of biological resources. 



292 Nutrients 



Influence of Nutrients on Animals 

As the great majority of animals display the holozoic mode of nutri- 
tion, this type of heterotrophism will be considered first, together with 
certain nutritional aspects of parasitism. The nutritional relations of 
saprophytes particularly the fungi and bacteria will be included 
in the discussion of decomposition in the succeeding section. 

The nutrients required by animals are proteins, carbohydrates, fats, 
salts, and accessory substances such as vitamins. Limitation by the 
nutrient factor may result from the shortage of necessary foods in 
the environment or from the inability of the animal to obtain and 
digest them. The deficiency of the food itself may occur in relation 
to its abundance or its composition. In some situations the total 
amount of nutriment is sufficient but the particles of food are so 
scattered that they cannot be gathered fast enough to satisfy daily 
nutritional needs. 

The main bulk of the animal diet consists of carbohydrates, fats, 
and proteins. Considerable flexibility exists in respect to the use of 
these materials as sources of energy, but at least some protein food is 
required in the growth and repair of animal tissue. Insufficient 
supply of food to meet energy requirements or deficiency in amount 
of protein, minor mineral nutrients, or necessary trace elements may 
act as a limitation on the growth, reproduction, or distribution of an 
animal. 

Free-living animals and parasites obtain these nutrient materials 
by feeding on the tissues of other organisms in their environment. 
The bodies of the organisms preyed upon as a source of food all con- 
tain carbohydrates, fats, and proteins, but in widely varying propor- 
tions, and they may or may not furnish all the minor and trace ma- 
terials that are needed. Furthermore, the possible food organisms 
must be caught, killed, and dealt with mechanically, in the process 
of ingestion, and chemically, in the process of digestion. 

Herbivorous animals may be either larger or smaller than the plant 
food on which they feed, but carnivores must always be larger, 
quicker, or stronger than their prey, or be able to overcome them by 
the use of poison glands or some other adaptation. Parasites are 
smaller than their hosts. Predators must also be equipped with ap- 
pendages and mouthparts that are capable of breaking through the 
bark of trees, the shells of invertebrates, the tough skin of verte- 
brates, or other types of protective outer covering, and capable of 
chewing up the food sufficiently for swallowing. Apparently soft 



Influence of Nutrients on Animals 293 

grass, for example, contains spicules of silica so hard that only rumi- 
nant animals with specially adapted teeth can feed on it as a regular 
diet. 

The necessity for alacrity and for the possession of suitable ana- 
tomical adaptation for dealing mechanically with the prey is suffi- 
ciently evident, but not so immediately apparent is the necessity for 
the animal to possess the digestive equipment required to deal chem- 
ically, with the food once it is inside. Carnivores do not live on hay 
nor do ruminants live on meat. In studying the food sources of 
animals the possible lack of enzymes for digesting materials that ap- 
pear to be abundant and available in the environment is a difficulty 
sometimes overlooked, especially in relation to lower animals. For a 
long time it was supposed that clams, oysters, and other common 
bivalves obtained their nourishment chiefly from the larger, more 
conspicuous species of diatoms filtered from the plankton. Most of 
these larger types of phytoplankton are now known to pass through the 
intestines of the shellfish quite undigested. Physiological investiga- 
tion has revealed the fact that these mollusks have no extracellular 
cellulase or protease. Hence, they are quite incapable of digesting 
relatively large food particles that are enclosed in an intact cellulose 
or protein cell wall ( Coe, 1948 ) . The nutritional needs of filter feed- 
ers like clams and mussels appear to be satisfied by the ingestion of 
organic detritus or of a type of nannoplankton small enough to be en- 
gulfed by digestive phagocytes. 

Many specializations have arisen in relation to the mechanical and 
chemical requirements of food getting in natural habitats ( Allee et al., 
1949, Ch. 17). Although most animals are clearly either herbivorous 
or carnivorous, these categories are not absolute and some species 
like the opossum are omnivorous in their diet. Some carnivores eat 
considerable plant material. Herbivores occasionally consume 
animal food, as is exemplified by the fact that reindeer have been 
known to eat fish when no other food was available. The diet of 
the black bear ranges all the way from sizable mammals to tiny ants, 
and from blueberries and honey to salmon. 

Many species of animals are highly modified for feeding on special 
parts of plants. The aphids and scale insects have sucking mouth- 
parts enabling them to obtain the sap of their hosts. Some insects 
are adapted for boring into and digesting, the wood, the bark, or the 
cambium of trees. Not only do certain insects feed exclusively on 
the leaves of plants, as do many mammals and a few reptiles, but in 
addition the "leaf miners," which live between the leaf surfaces, are 
further specialized to feed exclusively on the softer interior tissue. 



294 Nutrients 

Many animals, among which birds are prominent, show a predilection 
for the seeds or fruits of plants, whereas other types eat the flowers, 
or more frequently, only certain parts or secretions of the flowers, 
such as the pollen or the nectar. Hummingbirds, butterflies, and 
bees are well known in the latter category. Finally, the roots of plants 
form the chief or the exclusive food of some burrowing animals. As 
a further development of specialization, certain herbivores are re- 
stricted to a food from plants belonging to a particular taxonomic 
group, that is, to one family, one genus, or even one species. An ex- 
ample of selective feeding is shown in Fig. 6.15. 

Carnivores also may sometimes display an epicurean fastidiousness 
in their diets, but specialization on single animal foods is not as elab- 
orately developed as it is among herbivores. Certain species of bats, 
leeches, insects, mites, and ticks, to be sure, are specifically adapted 
for bloodsucking, and other instances of preference for certain animal 
tissues exist. Animals specially adapted for subsisting on dead or- 
ganic matter include the dung beetles and the carrion feeders such as 
vultures, flesh flies, and sylphid beetles. Various species of scaveng- 
ers are restricted to the lipoids, proteins, keratinoids, or tendinous tis- 
sues of the dead animal. For the most part, however, carnivores eat 
all parts of their prey and are guided largely by availability in their 
selection of food species. 

The distribution of free-living animals, parasites, and other hetero- 
trophic forms is obviously closely related to the occurrence of the or- 
ganisms upon which they depend for food. The nutrient factor limits 
geographical range in a general way or in a narrowly specific manner, 
according to the degree to which each species is omnivorous or re- 
stricted in its food habits. Grazing animals like the antelope are 
found in grasslands; browsing animals like the deer live in forested 
regions. Similar broad control of distribution occurs in the aquatic 
environment. Off the New England coast cod are caught chiefly 
on the banks, such as Georges Bank, where mollusks, crabs, and other 
organisms upon which the cod feeds grow abundantly on the firm 
bottom. In contrast, the redfish is found chiefly in the deeper water 
off the banks where the soft muddy bottom supports populations of 
shrimp which form the principal food of this fish (Fig. 8.4). 

Control of distribution on a smaller scale is seen in those hetero- 
trophic organisms that are limited to one type of food. Monophagous 
animals are those that eat only one species of food organism. Para- 
sites generally attack only a limited group of species sometimes no 
more than one species. Saprophytes are often capable of deriving 
nourishment from only one type of organic material, and, if this "sub- 



Influence of Nutrients on Animals 

T 



295 





FIG. 8.4. Contrasting distribuiton of cod, Gadus callarias (upper figure) and red- 
fish, Sebastes marinus (lower figure) as indicated by the catch of the New Eng- 
land commercial fishery in 1936. Each dot indicates a catch of 25,000 kg; broken 
line indicates 90 m contour. (Ackerman, 1941, Copyright, Univ. of Chicago 

Press.) 



296 Nutrients 

strate" is highly restricted in its occurrence, the distribution of the 
saprophyte will be correspondingly curtailed. 

Many illustrations of the control of distribution by the nutrient 
factor for these various types of organisms will occur to the reader, 
but perhaps the richest source of examples is found in the insect 
world, as fully discussed by Brues (1946). Our common North 
American walking stick, Diapheromera femorata, feeds generally on 
the leaves of oaks, whereas a giant East Indian member of this insect 
group eats the leaves of only a single species of Eugenia found in 
Sumatra. 

The degree to which insect pests on crop plants are monophagous 
is an important consideration in the introduction of new crop species 
and in the procedures to be adopted in pest control. The Colorado 
potato beetle, which spread northward from its native home in 
Mexico, feeds almost entirely on the foliage of the potato plant, and 
it sooner or later moves into every new area in which potato farming 
is begun. The cotton boll weevil crossed the Rio Grande from 
Mexico about 1892, and by 1894 it had spread to six counties in south- 
ern Texas. Advancing 40 to 160 miles a year, the weevil had infested 
more than 85 per cent of the Cotton Belt of the United States by 
1922. In view of the terrific destruction caused by this pest to the 
cotton crop, it is fortunate that the insect is prevented by its mono- 
phagous feeding habit from spreading to other crops or to native 
vegetation. 

Insects also provide examples of the high degree of specificity of 
many parasite-host relationships a matter in which nutrition is prom- 
inently involved. The Mallophaga (biting or bird lice) generally re- 
strict themselves to hosts of one, or of closely related, species. The 
physiological basis for this specificity is strikingly illustrated by the 
taxonomic relations of the lice that feed upon the bodies of the cow- 
bird. This bird lays its eggs in the nests of other birds belonging to 
no less than 158 species, where the young cowbirds would have every 
opportunity to acquire the lice of the foster species if the infestation 
depended primarily upon chance contact. Actually, however, the 
lice of the cowbird are not those of the foster species but belong to 
genera found on other blackbirds taxonomically related to the cowbird. 

The absence of certain minor constituents from the diet may have 
far-reaching ecological consequences. Lack of sodium chloride in 
the food causes deer, elk, and other ruminants to travel long distances 
to salt licks. Big Horn sheep, driven by the advance of civilization 
into parts of the Rocky Mountains where water is soft, must be pro- 
vided with extra salt. A rich supply of lime in the food is required 



Decomposition and Regeneration 297 

by snails, mammals, and certain other land animals. As mentioned in 
Chapter 3, especially fine race horses are raised in the bluegrass region 
of Kentucky. Here the abundance of calcium and phosphorus in the 
grass makes possible the healthy bone growth of the young horses. 
The fire salamander is an example of an animal that avoids calcium 
soils. The special need of aquatic organisms for lime has been dis- 
cussed in the previous chapter. 

Certain elements and vitamins are required by animals in only 
trace quantities, but, if these minute amounts are not present in the 
diet, serious deficiency diseases and eventually death result. In re- 
gions such as the belt from Washington east to Montana and south 
to Colorado, in which iodine is deficient in the soil, the plants, al- 
though themselves healthy, do not contain the amount of iodine 
needed by animals feeding upon them. As a result goiter develops 
in live stock, and also in human consumers, unless extra iodine in 
the form of iodized salt is added to the diet. 

Zinc, iron, copper, and cobalt are other elements needed in trace 
amounts by animals. Zinc is essential in one of the enzymes involved 
in respiration, and the other three elements mentioned are concerned 
with formation of hemoglobin, the deficiency of which produces 
anemia. In certain parts of Scotland and of Australia all attempts 
at sheep raising were unsuccessful until the difficulty was finally 
traced to the lack of cobalt in the soil. Only very small amounts of 
cobalt given the sheep directly, or applied to the grazing areas as 
fertilizer, were needed to bring about a remarkable improvement in 
the growth of the animals. 

The similar need of animals for vitamins obtained from the food is 
too well known to require extended comment here. Only minute 
quantities of vitamins are consumed, but these are absolutely neces- 
sary and must be obtained directly from the plants that manufacture 
them or in some other way, if the animals are to survive. Control of 
the lives of animals and of other heterotrophic forms by the nutrient 
factor may thus involve the sufficiency in amount of the common food 
stuffs, the availability of essential accessory substances in extremely 
small quantities, or some other necessary aspect of nutrition. 

DECOMPOSITION AND REGENERATION 

Since the food of animals and of other heterotrophic forms is de- 
rived from green plants or from the bodies of organisms that are di- 
rectly or indirectly dependent upon green plants, the nutrition of 
these phototrophic forms is clearly of basic importance in the natural 



298 Nutrients 

community. We have seen earlier in this chapter that for both ter- 
restrial and aquatic plants the nutrients that are likely to run short 
particularly phosphate and nitrate are derived in large measure 
from the decomposition of dead organisms. The destructive part of 
the biological cycle is thus revealed to be just as critical as the con- 
structive part in keeping the wheel of life turning. This idea can 
perhaps be expressed no more graphically than in the words of a 
student who wrote on an examination: "If it were not for the decom- 
position process, the whole world would become choked with the 
dead bodies of plants and animals, and this shocking situation would 
bring all life to an end." 

Processes of Decomposition and Transformation 

The release of the nutrients that have been built into the bodies of 
plants and animals involves two steps: first, the organic matter must 
be decomposed into soluble form and subsequently into inorganic 
form, and second, the resulting inorganic material must be trans- 
formed into compounds that can be absorbed by phototrophic plants. 
Carbohydrates decompose into carbon dioxide and water; fats break 
down into these materials and also release phosphate. The decom- 
position of protein is more elaborate, involving hydrolysis into pro- 
teoses, peptones, and polypeptides, and then break down into amino 
acids, ammonia, carbon dioxide, and water in addition to minor con- 
stituents. Special complexes present in living organisms, such as 
cellulose, hemicellulose, chitin, agar, and bone, decompose much more 
slowly but eventually to the same products. 

Living organisms are required to carry out practically every one 
of these steps in decomposition. Fungi are especially active in de- 
composing organic matter on land; bacteria are prominent in damp 
soil and in the water environment. These microorganisms not only 
work on the surfaces of solid material but also attack dissolved or- 
ganic matter. 

The necessity for the transformation of the products of decomposi- 
tion from one inorganic form to another is especially apparent in 
nitrogen compounds. Since most plants can obtain their nitrogen 
only in the form of nitrate, the ammonia or ammonium salts resulting 
from the decomposition of protein must be oxidized first to nitrite 
and then to nitrate. These transformations, comprising the process 
of nitrification, are brought about in both the land and the water en- 
vironments by the activities of autotrophic ( chemotrophic ) bacteria: 
the oxidation of ammonia to nitrite is accomplished by bacteria of the 



Decomposition and Transformation 299 

genera Nitrosomonas and Nitrosococcus, and the oxidation of nitrite 
to nitrate is carried out by bacteria of the genus Nitrobacter. 

The supply of available nitrogen in the soil and in the water may 
be further augmented as the result of nitrogen fixation by specialized 
bacteria belonging to the genera Azotobacter (aerobic) and Clos- 
tridium (anaerobic), which are free living, and Rhizobium, which 
live symbiotically in root nodules on certain higher plants, notably: 
locust trees, alfalfa, clover, beans, peas, and other legumes. These 
bacteria take in free nitrogen, build it into their bodies and, in the 



Nitrate ( N0 3 ) salts -*- 



Plant proteins 
and other com- 
plex nitrogen 
compounds 



Animal proteins 
and other com- 
plex nitrogen 
compounds 



Nitrogen compounds 

in dead organisms 

also urea and 

other wastes 



Proteins and 
other complex 
organic com- 
pounds of pro 
toplasm 




NH 3 
ammonia 

FIG. 8.5. Diagram of the nitrogen cycle, showing the principal components and 

processes. Compounds not included in circles are free in the environment. 

(Modified from Principles of Modern Biology by Marsland and Plunkett. By 

permission of Henry Holt and Co., Copyright, 1945.) 

case of the symbiotic forms, pass it on to their hosts. After these 
bacteria or the tissues of the host plants die and decompose, the sup- 
ply of fixed nitrogen that they contain is made available through the 
activities of the nitrifying bacteria. Free nitrogen can also be fixed 
by certain photosynthetic bacteria, sulfate-reducing bacteria, and 
blue-green algae. Other types of bacteria cause denitrification, or 
the loss of fixed nitrogen from the environment, by reducing nitrate 
to nitrite and nitrite to nitrogen, or even to ammonia (Frobisher, 
1944, Ch. 25; Welch, 1952, Ch. 10). 
Thus nitrogenous material derived from dead animals and plants 



300 Nutrients 

is decomposed and transformed by different microorganisms into in- 
organic compounds that can be utilized once more in the growth of 
phototrophic plants. The various stepssome of them reversible- 
involved in this vital nitrogen cycle are indicated in Fig. 8.5. An 
example of the quantitative changes in successive nitrogenous prod- 
ucts is shown in Fig. 8.6 depicting the course of an experiment in 



400 r- 




5 10 15 20 25 30 35 40 45 50 55 

Days 

FIG. 8.6. Decomposition of nitrogenous organic matter in mixed plankton under 
laboratory conditions, showing the successive appearance of soluble nitrogen com- 
pounds. One portion inoculated with diatoms after 45 days; dotted lines represent 
uninoculatcd portion. (Von Brand, Rakestraw, and Renn, 1937, Biological Bull) 

which marine plankton decomposed and a new growth of the plank- 
ton took place after regeneration of the nutrients. Phosphorus, car- 
bon, sulphur, and other elements entering the living complex take 
part in similar cyclic movements between the organism and the en- 
vironment. The carbon cycle is represented diagrammatically in 
Fig. 8.7. 

Place of Decomposition 

In the terrestrial environment the decomposition of organic matter 
takes place on and in the soil. Material in particulate form is carried 
beneath the surface by organisms and by rain to depths varying ac- 
cording to the ecological conditions as discussed in Chapter 3. Mate- 
rial in solution may be transported still further by percolating water. 
The decomposition of this organic matter thus may take place at a 
variety of levels within the soil profile, although most of it goes for- 
ward in the A horizon. As we have mentioned earlier, a good por- 
tion of the products of decomposition may be retained in the soil 
when the vegetation is suitable and the rainfall moderate, but some 



Place of Decomposition 301 

loss of mineral salts takes place under the best of conditions. How- 
ever, serious amounts of these valuable nutrients are leached from 
soils that are acid and subject to excessive rainfall or that exist un- 
der other unfavorable circumstances. Farm or forest practices that 
permit soil erosion to take place have resulted in accelerated and often 
irreparable loss of mineral nutrients and humus from the land. 



Respiration 



Carbon dioxide 
in air and natural waters 



Respiration 



Carbon (organic) 

compounds of 

plant protoplasm 

carbohydrates 

proteins 

lipins 

etc. 



Organic compounds 
of animal proto- 
plasm 



Absorption Metabolism 




?aras/fe s 



Organic compounds 
of protoplasm 




Carbon (organic) 
compounds in the 

environment 
in dead organisms 

and their 

remnants 



Organic compounds 
of protoplasm 




FIG. 8.7. Diagram of the carbon cycle, showing the principal components and 

processes. Compounds not included in circles are free in the environment. 

(From Principles of Modern Biology by Marsland and Plunkctt. By permission of 

Henry Holt and Co., Copyright, 1945.) 

In the water environment dead organisms tend eventually to sink 
even though they may have a period of buoyancy. After the or- 
ganism's substance has gone into solution, no further tendency to sink 
exists. During the period of sinking the decomposing material may 
have been displaced horizontally by currents for considerable dis- 
tances. By the time that the organic matter has been decomposed 
and transformed, the resulting nutrient materials may be very far re- 
moved both horizontally and vertically from the spot originally in- 
habited by the organism from whose body they were derived. 

In deep areas of lakes or of oceans the bodies of many dead plants 
and animals sink to depths below the euphotic zone before they 



302 



Nutrients 



decompose. Under these circumstances, when the phosphate and 
nitrate are released in form suitable for absorption by green plants, 
they will be at levels where light is too weak for photosynthesis. 
These nutrients cannot be used again for plant growth until they have 
been returned to the euphotic zone by vertical currents and mixing, 
as will be discussed in the next section. A cycle thus exists involving 
this movement of nutrient materials to the deeper parts of water 
bodies and the restoration of the decomposition products to the upper 



1000 



I 

E - 2000 

fc 
& 



3000 



4000 




LL 



.// i 



4.0 



1.0 2.0 3.0 

P0 4 -P, Mg-atoms/Z, 

FIG. 8.8. Vertical distribution of phosphate in the Atlantic, Pacific, and Indian 
Oceans. (From The Oceans by Sverdrup et al., 1942, Copyright, Prentice-Hall, 

N. Y.) 

layers by vertical transport. In deep lakes and in the ocean the con- 
centration of nutrient salts is much greater in the lower strata than it 
is near the surface, as may be illustrated by the condition in the 
Pacific (Fig. 8.8). The rates of upward and downward transport for 
the year as a whole are presumably in approximate balance, and the 
cycle tends toward a steady state. However, mineral salts are con- 



Stagnation in Cycles 303 

tinually being added to the ocean by run-off and river discharge a 
process constituting a loss of nutrient material from the land, These 
nutrients enter the marine cycle and the major portion of them is 
eventually distributed to the huge reservoir of nutrients in the deep 
sea. These critically important materials are thus being drained 
away inexorably from the land environment and being added to the 
accumulation in deep water which is unavailable, except for that por- 
tion brought to the surface again by vertical circulation. 

Stagnation in Cycles. Materials move from the environment into 
the bodies of plants and animals as they grow, return to the environ- 
ment when they die and decompose, and in some instances undergo 
complicated transformations and translocations in the environment 
before they are again taken up by living organisms. The study of 
the geographical distribution of materials used by plants and animals 
and their cycles is known as biogeochemistry. A further discussion 
of these circular causal systems in ecology is presented by Hutchin- 
son (1948). Materials are not equally abundant in the different 
phases of these cycles, nor do the various steps take place at uniform 
rates. Biogeochemical investigations show that materials of critical 
concern to plants and animals have accumulated in certain places and 
represent points of stagnation in the cycles. The gradual augmenta- 
tion of nutrient materials in the deep sea is an instance of this phe- 
nomenon. Nutrient substances may thus be withdrawn from circula- 
tion for longer or shorter periods. 

A relatively temporary stagnation is represented by the organic 
matter in the soil or on the bottom mud of natural water bodies. The 
nutrient minerals contained in this organic matter are unavailable for 
plant growth until the material decomposes. In many soils this re- 
tardation of the cycle is beneficial because, as explained earlier, the 
formation of humus in the soil has valuable physical effects besides 
providing a slow, steady release of nutrients. Guano deposits, such 
as those on the islands off Peru, illustrate a long-term accumulation 
of nutrient materials (Hutchinson, 1950). The many cubic miles of 
peat, lignite, and coal buried in the earth's crust represent stagnation 
in the carbon cycle for hundreds of millions of years. A similar 
stagnation in the calcium cycle is represented by the deposits of chalk, 
limestone, and coral material. 

Great differences exist in the total supply of the various building 
materials needed by plants and animals as well as in their availability. 
When stoppages occur in cycles of elements likely to be scarce in 
available form, such as nitrogen and phosphorus, critical conditions 
may arise. An inexhaustable supply of nitrogen exists in the earth's 



304 Nutrients 

atmosphere, but this represents a reservoir that can be drawn upon 
only slowly by certain physical and biological agents. Phosphorus, 
on the other hand, is a rare material: only about ^oo f * ne earth's 
crust is composed of this element. The only ready sources of phos- 
phorus are the products of decomposition of the bodies of organisms. 
We have seen that the supply of phosphate in the rocks and in the 
deep sea can be obtained at only a very slow rate. Therefore the 
acceleration of the loss of this element from the land by soil erosion 
is a critically serious matter for life in general, and for our agriculture 
in particular, since our food supply is chiefly derived from the ter- 
restrial environment. Wells, Huxley, and Wells (1939) have said: 
"Phosphorus is the weak link in the vital chain on which man's civili- 
zation is supported." 

Regeneration 

The return process in the cycle of materials in natural environ- 
ments, by means of which nutrients once used are made available 
again for the further growth of organisms, is known in ecology as 
regeneration. There are two aspects of regeneration: first, nutrients 
must be rendered available chemically through the processes of de- 
composition and transformation that we have traced in the preceding 
sections; second, nutrients must be rendered available spatially, that 
is, restored to zones where green plants can grow and resynthesize 
organic material. Critical minerals that have been carried below the 
level of plant growth must be brought up again before they can be 
used. 

Mention has been made earlier of the favorable situation in prairie 
soils in which rainfall is not sufficient to cause serious leaching, and 
in semiarid soils in which an upward movement of the ground water 
takes place. These effects, augmented by the activity of the roots 
of grasses, account to a considerable degree for the restoration of 
nutrients to the surface layers in pedocal soils. Regeneration of 
nutrients in the pedalfer soils is less complete. In the deciduous 
forest region, however, some restoration of materials to the surface 
is accomplished by the fall of leaves and other litter, and by the min- 
ing activities of earthworms and other animals. In the podzol re- 
gions the greater rainfall produces excessive leaching with the result 
that nutrient salts are carried beyond the reach of plant roots. 

Here we have an analogous situation to that in the deep parts of 
the aquatic environment, where decomposition is often completed 
at depths below those at which green plants can grow. In water 



Rate of Regeneration 305 

habitats the return portions of the same currents that carry oxygen 
down to the deeper levels in lakes and in the ocean bring nutrients 
up from the richer reservoirs in the lower strata. This upward trans- 
port of nutrients by wind stirring, eddy conduction, or mass circula- 
tion which also affect other properties of the water is another mani- 
festation of the principle of common transport. 

Rate of Regeneration. The rate at which regeneration takes place 
varies very widely and depends not only upon the speed of decom- 
position but also upon the rapidity of restoration of nutrients to the 
growth zone. The rate of regeneration is rapid in some instances 
and slow in others. The rapidity with which the processes of decom- 
position and transformation go forward depends upon temperature, 
supply of oxygen, and other conditions that influence the industrious- 
ness of the various kinds of bacteria required. In very cold regions 
and in poorly aerated soils and muds the chemical steps in regenera- 
tion take place at a sluggish rate and sometimes come to a standstill. 
Under these circumstances undecomposed, or partially decomposed, 
organic matter accumulates and, if time and circumstances are right, 
deposits of peat, coal, or oil may be formed. 

When decomposition and transformation to inorganic compounds 
have taken place, the rate at which nutrients are restored to surface 
layers depends on the physical conditions in the soil and in the water. 
In oceanic areas of active upwelling of water from deeper layers the 
regeneration of nutrients may take place as fast as the plants can use 
them. The extensive regions of permanent upwelling off the west 
coast of Africa and the west coast of North and South America are 
characterized by water rich in nutrients, and consequently they sup- 
port a vigorous plant growth at all seasons. In most temperate waters 
vertical circulation is retarded or stopped completely during the sum- 
mer. The deep stirring in temperate seas during the winter period 
brings a new charge of nutrient-rich water to the surface and gives rise 
to the expression: "Once a year the sea is plowed." In deep lakes of 
the temperate region phosphate and nitrate are restored to the euphotic 
zone during the spring and fall overturns. 

During periods when virtually no vertical circulation takes place 
plant growth in natural waters must depend upon the nutrients fur- 
nished by decomposition within the surface layers and by the rela- 
tively slow eddy conduction from deeper strata often further retarded 
by the presence of a thermocline. An adequate rate of replacement 
is just as necessary as an adequate concentration of nutrients. A 
single measurement of the phosphate and nitrate in the water might 
indicate the presence of adequate amounts of these substances, but 



306 



Nutrients 



in view of the rapidity with which the nutrients are used by a grow- 
ing plant population, the supply of nitrate and /or of phosphate will 
soon be exhausted unless regeneration keeps pace with utilization. 
Quantitative data on the rates of decomposition, regeneration, and 
assimilation of phosphate by phytoplankton are available from con- 
trolled experiments in outdoor tanks (Pratt, 1950) in addition to lab- 
oratory tests (Rice, 1949). Analysis of the seasonal cycle of phos- 
phate in the Gulf of Maine by Redfield, Smith, and Ketchum (1937) 
gives the following approximate estimates for the fractions of this 
nutrient assimilated by the phytoplankton from the various sources 
of supply: 2 per cent from the original inorganic phosphate in the 
surface water layer, 73 per cent from vertical transport, and 25 per 
cent from decomposition and animal excretion within the surface 
layer (Ketchum, 1947). Similar experiments in the terrestrial en- 
vironment have been carried out principally in relation to cultivation 
of crops, and relatively little is known about the rate of regeneration 
in wild areas. For further information on this subject for cultivated 
areas the reader may consult the Yearbooks of the United States De- 
partment of Agriculture. 

Ratio of Regenerated Materials. The proportions of the materials 
provided by the regeneration process are limited by the relative abun- 




50 100 



100 150 200 



50 100 50 

Phosphate (P 2 6 ) 

FIG. 8.9. Correlation between concentrations of nitrate and phosphate in waters 

of the Atlantic, Indian, and Pacific Oceans expressed in milligrams per cubic meter 

( Redfield, 1934, Copyright, Univ. Press of Liverpool. ) 



Ratio of Regenerated Materials 307 

dance of the elements in the organic matter that has decomposed. 
Only those nutrients will be provided by decomposition that were 
present in the dead organisms. The fact that chemical fertilizers 
added to the soil do not replace all the nutritive substances, nor fur- 
nish them in the same proportions as those removed by the growth 
of the farm crops, has already been mentioned. Procedures recom- 
mended by "organic farmers" in which the unused portions of the 
crops are returned to the soil as a green manure and in which animal 
manure is used extensively go far toward maintaining a proper bal- 
ance of organic matter and the various mineral nutrients, but further 
correction of proportions or the addition of trace elements may also 
be necessary. 

The interdependence of the ratios of materials regenerated and 
those of nutrients assimilated is also demonstrated in the aquatic en- 
vironment. Analysis of samples of sea water from many localities in 
the Atlantic, Indian, and Pacific Oceans reveal a striking parallelism 
in the ratio of phosphate and nitrate in spite of the great fluctuations 
in the concentration of each (Fig. 8,9). The ratio of carbon to the 
other two elements also tends to remain constant, giving the follow- 
ing average values: 

C:X:P = 41:7:1 grams - 106:15:1 atoms 

The ratios of these three elements in the plankton are found to be 
very closely the same. Plankton is thus known to take up nutrients 
in the ratio in which they are provided in the water; when the plank- 
ters die, these materials are restored to the water in the same ratio 
(Redfield, 1934). It is not easy to decide how this unique situation 
arose. Do these ratios represent the composition of the primitive 
ocean in which species of phytoplankton evolved that could assimilate 
nutrients in just these proportions? Or do the ratios represent an 
equilibrium reached and maintained by the activity of nitrogen-fixing 
and denitrifying bacteria in the sea? The answer to these questions 
must await further investigation. 

Let us cast a backward glance over the web of interdependencies 
involved in the nutritive relations between the organism and the en- 
vironment. We have seen that the lack of nutrients in quantity or 
quality, or in chemical or mechanical availability, limits the growth 
and distribution of both plants and animals. As living things grow, 
they reduce the supply of nutrient materials in their environment; 
but, when they die and decompose, their substance adds to this sup- 
ply directly or indirectly. In situations in which either the organ- 
isms or the nutrients are mobile the shortage of food affects the whole 



308 Nutrients 

population, not merely the individuals most recently added to the 
population. For a replenishment of certain critical nutrients the chief 
source is the dead bodies of the animals and plants themselves. The 
presence of other sets of organisms the decomposers and transform- 
ersis required to convert the organic materials to forms suitable for 
the growth of green plants, and frequently physical agents are neces- 
sary to restore the nutrients to zones where growth can take place 
once more. 



9 

Relations within 
the Species 



Our discussion of nutrients in the previous chapter leads directly to 
a consideration of the presence of other organisms as a part of the 
environment. No individual animal could live by itself because it is 
dependent upon other organisms for food as well as for other require- 
ments. Certain green plants could conceivably exist in isolation for 
a period, but, if the species is to be maintained, progeny must be 
produced. The plant would soon be surrounded by young with 
which it would be in competition, or in some other relation, so that 
it would no longer be living as an isolated individual. As the popu- 
lation of each species increases and as groups of animals and plants 
are formed, new relations appear in ecological situations as well as 
in others the whole is more than the sum of its more obvious parts. 
In this chapter we shall deal with intraspecific relationships and in the 
next chapter with interspecific relationships. 

ORIGIN OF GROUPS 

Groups of individuals of the same species may arise in several ways: 
(1) as the result of reproduction, (2) as the result of passive trans- 
port, or (3) as the result of active locomotion. 

Reproduction 

Some groups are the result of the breeding activity of one breeding 
uniteither an individual in the case of asexual reproduction, or a 
pair in the case of sexual reproduction. If the progeny of this breed- 
ing unit stay near together, a group will be formed. In some types 
of animals and plants the young remain attached to the adults. This 
situation is seen commonly in the cryptogamic plants, but instances 
are also found among the higher plants. Strawberry plants, for ex- 

309 



310 Relations within the Species 

ample, send out runners that take root, producing new plants, and 
these in turn send out more runners. In this way dense aggrega- 
tions of the species are formed. Unfortunately for man, briars and 
poison ivy also produce thick clumps of individuals by this very suc- 
cessful vegetative reproduction. 

Similar attached aggregations are formed by certain sessile organ- 
isms of the animal kingdom. Many sponges, bryozoans, tunicates, 
hydroids, and other invertebrates grow in colonies, and coral animals 
produce extensive formations as a result of colonial development. 
Animals displaying this growth habit are sometimes very numerous, 
but they are not capable of free locomotion, and they have not at- 
tained as high a degree of development as non-colonial animals. 

In other groups formed by the progeny of a breeding unit the in- 
dividuals are unattached but remain together. The parents and their 
immediate offspring constitute a family among animals, and the same 
term is sometimes applied to the progeny of a single plant. A few 
relatives may join the family unit, as has been observed in a den of 
wolves (Murie, 1944). Larger aggregations result from a further 
extension of family groups. An animal and "his sisters and his 
cousins and his aunts" may remain together as a clan, and unrelated 
members of the species may subsequently join the group to. form a 
larger herd, pack, or flock. Seals, sea lions, wolves, monkeys, prairie 
dogs, many ungulates, and various kinds of birds are examples of ani- 
mals that form units of this sort. More complex groups of related in- 
dividuals are represented by colonies of social insects, such as ants* 
bees, wasps, and termites. In the plant kingdom the progeny of 
neighboring adults may join to form larger groups of the same 
species. If such a group is invading a bare area, it is termed a plant 
colony, as will be discussed more fully in Chapter 12 in relation to 
ecological succession. 

In other instances groups resulting from reproductive activities are 
not necessarily the descendants of one breeding unit. Aggregations 
of young are frequently formed by the simultaneous release of eggs 
or larvae within a restricted area. The coincident production of 
young by neighboring adults is often set off by some common en- 
vironmental stimulus, such as the occurrence of a critical illumination 
or temperature. The nearly simultaneous spawning of oysters, trig- 
gered by temperature conditions as described in Chapter 5, results in 
the sudden appearance of swarms of oyster larvae in estuaries of the 
Atlantic coast. 



Passive Transport 311 



Passive Transport 

A second, and entirely different, mode of origin of groups of the 
same species results from the passive transport provided by the 
medium. Wind often sweeps mosquitoes and other insects from ex- 
posed terrain and thus indirectly concentrates them in sheltered places. 
Most commonly this mechanical action of the medium in bringing 
about aggregations is seen in the aquatic environment. Inhabitants 
of streams tend to be swept together in eddies, and the currents of 
lakes and oceanic areas often concentrate phytoplankton and zoo- 
plankton in the same way, but more slowly. The current system in 
the Gulf of Maine, for example, plays a major role in the accumula- 
tion of certain types of plankton in the water mass overlying Georges 
Bank. The clockwise circulation of water around the margin and 
the relatively quiet eddy of homogeneous water over the center of the 
Bank have produced conditions in the central area that favor the de- 
velopment of Sagitta and of the copepod fseudocalanus but are un- 
favorable for Catanus, a closely related copepod. The current sys- 
tem tends to concentrate the two former species in the central part 
of the Bank and to exclude immigrants from populations of Calanus 
and other species developing beyond the margins of the Bank ( Clarke, 
Pierce, and Bumpus, 1943). 

Another example of the concentration of animals by passive trans- 
port is the occurrence of "plankton traps" found particularly in Scandi- 
navian fjords but also to a lesser extent in other estuaries. Charac- 
teristically, a considerable amount of fresh water enters from rivers 
at the head of the fjojd, and this flows out at the surface over the 
more saline water beneath. Water from offshore moves in at the 
bottom, gradually rises at file head of the fjord, and mixes with the 
outward flowing water. Plankton organisms that tend to remain in 
deeper water layers because they are positively geotactic or negatively 
phototactic are drawn into the fjord by the deep circulation but are 
not carried out again by the surface flow. The result is that a me- 
chanical concentration of the plankton takes place in the lower layers 
of the f jordy In any species in which tactic responses cause the young 
stages to move toward the surface, a horizontal separation of the age 
groups results from this current actiqn. The transport of the larvae 
of sessile invertebrates to the upper portions of tidal estuaries, as de- 
scribed in Chapter 2, is another manifestation of the concentrating 
action of this type of differential water movement. 



312 



Relations within the Species 



Active Locomotion 

Groups of the same species brought about by the active locomotion 
of the individuals may arise ( 1 ) from the guidance of the organisms 
towards the same area by oriented responses to inanimate features 
of the environment or (2) from the attraction of the organisms to 
others of their own kind. The formation of groups by active locomo- 
tion is, of course, found most frequently in the animal kingdom, but 
certain motile algae and the swarm spores of aquatic plants also exhibit 
this behavior. 

Common Orientation. If the individuals of a species react in the 
same way to some physical stimulus in the environment, their locomo- 
tion will bring them to the same locality with the result that an aggre- 
gation will be formed. A familiar example is the clustering of insects 
about a source of light at night as a result of their positive phototaxis, 
or the attraction of fish and invertebrate animals to a torch held over 
the water (Maeda, 1951). Since land isopods, such as the wood 
louse, move more slowly, or stop creeping entirely, under moist con- 
ditions they tend to collect in damp places. A dead fish on the shore 
or a dead deer in the forest acts as a lodestone to which a large num- 




FIG. 9.1. Aggregation of mud snails (Nassa obsoleta) exposed at low tide in 
Barnstable Harbor, Mass. 



Common Orientation 



313 



her of scavengers will be attracted. Other sources of food or shelter 
similarly serve as a focal point at which animals from the surrounding 
areas tend to congregate (Fig. 9.1). In these instances each individ- 
ual reacts independently, and the group is formed as a secondary 
consequence. 

Sometimes the aggregation is the result of more complex and long- 
continuing reactions. Several kinds of marine worms form aggrega- 
tions by reacting individually to some aspect of the lunar cycle and 
swimming to the surface of the sea, as described in Chapter 6. When 
the animals have formed a huge swarm in this way, they initiate their 
breeding activity. Since the eggs and sperm are thus discharged 
into the water at the same time and at close quarters, the chances of 
successful fertilization are much improved. Consequently, the re- 
action of the animals to the physical stimulus that leads to the forma- 
tion of the aggregation has survival value in this instance. 

The dense schools of salmon preparing to breed in the headwaters 
of streams have come into being following the orientation of each in- 
dividual separately during the previous weeks while the fish was find- 
ing its way for hundreds of miles up from the ocean (Fig. 9.2). 




Photo International Pacific Salmon Fisheries Commission 

FIG. 9.2. Sockeye salmon in Adams River, British Columbia, schooling near bank 
before moving onto the spawning beds. ' Note dead, spawned -out salmon washed 

up on the bar. 



314 Relations within the Species 

When a large number of salmon have arrived at a suitable spawning 
place, the fish react more definitely to each other, pair off, and begin 
their breeding activity. After spawning the adults of the Atlantic 
salmon eventually return to the sea, but in the Pacific species the 
adults die in the stream and huge numbers of dead fish may choke 
the narrow waterways. The great quantity of decomposing organic 
matter thus added to the streams may deplete the oxygen supply 
locally and produce further ecological consequences. Nutrient mate- 
rials resulting from the decomposition may possibly stimulate the 
growth of plankton on which the young fish depend. When the eggs 
hatch, a large number of young fish appears simultaneously in the 
headwaters of each salmon stream and represents another occurrence 
of aggregation in this species. A further discussion of the ecological 
problems involved in the life history of the salmon will be found in 
a symposium sponsored by the American Association for the Advance- 
ment of Science (1939). 

Mutual Attraction. Another manner in which aggregations of the 
same species are formed is by an initial direct attraction of the in- 
dividuals to each other. Mutual attraction of individuals is found 
among lower animals, but frequently the reaction is largely non- 
specific. If isopods are distributed over a surface of uniformly low 
moisture, the animals stop against the first individual to come to a 
halt and soon a dense cluster will be built up. Brittle starfishes placed 
in a bare aquarium will move to form closely entwined aggregations, 
but, if the aquarium contains eel grass, or even glass rods simulating 
eel grass, the starfishes will remain spaced out in contact with these 
objects. In these instances the other individuals present are merely 
satisfying physical needs of moisture or of contact. In schools of 
fish, flocks of birds, and herds of mammals the origin of the group 
results from mutual attraction on a highly or a completely specific 
basis. "Birds of a feather flock together" because of a definite attrac- 
tion to others of the same species. The members of these groups 
are not necessarily from the same parents and are usually quite un- 
related, but during the breeding season units are formed within some 
populations in which the members are breeding partners and family 
relatives, as discussed above. 

Sometimes the reaction to keep in close contact, or to follow others 
of the same species, is very strong indeed. The manifestation of this 
in groups of ungulates, such as sheep, has given rise to our term 
"sheep-minded." Anyone who has visited sheep country knows the 
very great strength of the tendency of these animals to keep close 
together. So strong is this reaction that if the leaders of a flock 



Mutual Attraction 315 

stumble in a ditch, the remaining sheep will continue crowding for- 
ward causing a "pile-up." Sheep men greatly fear the occurrence 
of these pile-ups in which as many as 500 sheep may be killed within 
a few minutes. 

Animals forming groups recognize others of the same species most 
commonly by vision, but various other means are also used. Birds 
migrating in darkness or fog, or moving through thick vegetation, 
probably rely on call notes for keeping in touch with one another. 
Sound is probably used in the aquatic environment also, to a much 
greater extent than has been realized. Sound travels through the 
water medium much more effectively than through the air medium 
quite the reverse of the relationship for other orienting stimuli, such 
as light and odor. Various crustaceans, a great many fishes, and cer- 
tain cetaceans, including blackfish and porpoises, produce grunts, 
whistles, squeaks, and other noises that can be heard under water 
for great distances and are undoubtedly useful for recognition among 
aquatic animals (Kellogg, Kohler, and Morris, 1953). 

Some species of ants which are totally blind nevertheless manage 
to keep close together by the use of their "contact odor" sense. In 
tests with bullhead fishes each individual was found to move toward 
any object of the same size and color as itself. The fish would then 
touch the object with its barbels, and, if the sense organs encountered 
a paraffin model, the fish would move away. In this instance the first 
reaction causing the fishes to move together was a visual one; other 
reactions came into play subsequently. Among land vertebrates 
many mammals use odor for recognition. 

Sometimes the reaction to school or to form flocks is so strong that, 
if no others of the same species are available, the animal will join a 
group of another species. Thus we commonly find a few isolated 
gulls flocking with a group of terns on the shore, or a single sander- 
ling hurrying along the beach in company with a flock of least sand- 
pipers. Examples among other types of animals will occur to the 
reader. An amusing illustration of this tendency to associate with 
a group, even of another species, was observed in the large basement 
tank at the laboratory of the Woods Hole Oceanographic Institution. 
A number of squid had been kept in this tank for some time, and 
these animals always moved together in a dense school. One day a 
small mackerel of about the same size was placed in the tank. Since 
no other mackerel were present, this fish immediately joined the ranks 
of the squid, swimming along with them at the same rate in close 
formation. As the observer approached the side of the tank, the 
squid were startled and shot backwards. Since the mackerel was 



316 Relations within the Species 

quite unable to move in this direction, he was left isolated and ap- 
parently bewildered, turning to right and left. Never was there such 
a frustrated mackerel! 

The groups that have arisen in the various ways described above 
show all gradations of integration and permanency. In brief, we 
may note that some groups are brought together mechanically and 
often the individuals have no direct relation to one another. In other 
groups, such as insect colonies, a definite organization of the mem- 
bers exists. Some of the aggregations are of a temporary nature, 
lasting for only a few hours, days, or weeks. Starlings are commonly 
observed to gather in noisy flocks in the evening and to roost together 
in certain large trees, belfries, or other favorite spots; but the birds 
usually disband at daybreak, flying about independently or in small 
groups during the daylight hours. Bats follow a reverse daily sched- 
ule, coming together to roost during the day and dispersing during 
the night. Some aggregations exist only during the breeding season, 
whereas in other species flocking or herding takes place for the period 
of migration only. On the other hand, many herds of large mammals 
and many insect colonies are essentially permanent outlasting the 
lives of individual members and remaining in existence until de- 
stroyed by some unusual environmental change. For a fuller dis- 
cussion of this subject and of other aspects of animal aggregations, 
the reader is referred to the more extended treatment by Allee 
(1931). 

EFFECTS OF INCREASED NUMBERS 

An increase in the abundance of a species originating in any of the 
ways discussed in the previous section results in consequences of 
concern to the species itself and also to other interdependent species 
of the habitat. The repercussions from the numerical increase of one 
species on other species will be taken up in subsequent chapters. 
Here we shall consider the results of population increase within the 
species, dealing first with relations that are generally harmful and 
latej* with beneficial relations. Our present discussion will be cen- 
tered on contemporary effects, that is, on effects within the life span 
of the individuals or of the populations considered. Long-term ef- 
fects on the species may be quite different. Competition, for ex- 
ample, is usually harmful to the individuals concerned, but its selec- 
tive action in guiding the course of evolution may result in an eventual 
benefit to the species as a whole. 



Harmful Effects 317 



Harmful Effects 

An increase in numbers means an increase in competition for the 
necessities of life. Rivalry between members of the same species is 
typically keener than that between members of different species. 
In the words of Darwin (1859), "the struggle will almost invariably 
be most severe between the individuals of the same species, for they 
frequent the same districts, require the same food, and are exposed 
to the same dangers." Overpopulation therefore results in a serious 
interference of one individual with another sometimes in a passive 
or indirect way, at other times as direct aggression or even cannibal- 
ism. The accumulation of metabolites often curtails the further 
growth of the species within the area, and the simple matter of oc- 
cupying available space imposes a mechanical limitation on some 
populations. Since established land plants are fixed in position, the 
sphere of influence of each individual, and the area from which it 
must draw its necessities, are sometimes easier to observe than those 
of active animals. When a population of plants has increased so that 
the individuals are growing close together, their roots compete for 
nutrients and for water and their tops compete for light. As com- 
petition becomes more intense, growth rate is correspondingly re- 
tarded. 

A ready-made record of the effects of competition was found in 
the cross section of the trunk of a locust tree that had grown in Bel- 
mont, Mass., and was blown down in the hurricane of 1944 (Fig. 
9.3). By counting growth rings the date of the first year of the tree's 
life was determined as 1929. At that time an open field was aban- 
doned and a few trees seeded themselves in the area at widely spaced 
intervals. For the first few years thereafter, the tree grew rapidly, 
but, as size increased, competition with neighboring trees for nutrients, 
for water, and particularly for light became progressively more se- 
rious. The result may be observed in the diagram as progressively 
smaller growth rings in 1934, 1935, and 1936. In the spring of 1937 
the land was cleared of most of the trees for the construction of a 
house. With competition removed the growth of this locust tree 
was "released," to use the term of the forester. More wood was 
added during the growing season of 1937 than had been added in 
any of the previous 4 or 5 years, and growth continued at a high rate 
for the remainder of the life of the tree. 

The form of development of plants as well as their growth is af- 



318 Relations within the Species 

fected by competition. The growth form of a solitary tree is com- 
pletely different from that of a tree of the same species that has de- 
veloped in a stand closely surrounded by other individuals. In the 
latter situation the lower branches are killed by the reduction of light, 
or the growing buds and branchlets are knocked off by the branches 
of trees near by, as they whip about in the wind. The smaller, 
neighboring trees that bring about this pruning action are referred 
to as "trainers" by foresters, since they cause the tree to develop 
without lateral branches in a form that will later be suitable for saw 
timber. 

^A^ ^tet ^ 

.Growth ring for 
the season of 1944 

Inner bark 

Outer bark 




Land cleared 
spring 1937 
Growth 
released 



Progressive 
overcrowding 
Competition 
retards growth 



FIG. 9.3, Cross section of locust tree (Robinia Pseudo- Acacia), 17 cm in diam- 
eter, grown in Belmont, Mass., showing effects of early overcrowding and sub- 
sequent "release." 

Many examples of the harmful effects of overcrowding could be 
cited for other types of plants and for animals. We find the same 
principles of competition at work among populations of microbes as 
among forest trees. Bacteria deplete their supply of nutrients as they 
multiply and produce an accumulation of metabolites, until the fur- 



Harmful Effects 319 

ther growth of the colony is prevented. Organisms continuing to live 
in the same medium tend to change or to "condition" it. Condition- 
ing of flour by laboratory populations of the flour beetle Tribolium 
confusum has been shown to cause reduced fecundity, extended dura- 
tion of metamorphosis, and increased mortality (Park, 1941). The 
same kinds of harmful changes are brought about by overcrowding 
under natural conditions. In some situations aquatic animals may 
aggregate to such an extent as to exhaust the oxygen supply; else- 
where they may exhaust the food supply, produce harmful metabo- 
lites, or displace one another mechanically. Of several hundred 
oyster larvae originally setting on an old shell or other small object, 
only a few will find sufficient space to develop into full-sized oysters; 
all the others will eventually be killed by crowding. Where com- 
petition for space is keen, oysters grow in a long slender form, un- 
desirable for the market. Oystermen in cultivating their beds com- 



$ 



22 
20 
18 
16 
14 
12 



I I I I I I I I I 




I I 



10 20 30 40 50 60 70 80 90 

Mean flies per bottle 

FIG. 9.4. Curve showing decrease in rate of reproduction in Drosophila as cul- 
tures become more crowded. ( Pearl and Parker, 1922. ) 



320 



Relations within the Species 



monly break clusters of young oysters apart to insure that the animals 
develop in the well-rounded form characteristic of solitary oysters. 

The harmful effect of excessive numbers of animals is sometimes 
manifested as an interference with breeding. The progressive re- 
duction in the reproductive rate of Drosophila with increased density 
of the population is shown in Figure 9.4. As the flies become more 
crowded, interference with feeding and possibly also with oviposition 
is experienced. Frequent collision and interruption of feeding re- 
sult in inadequate nourishment and lowered fecundity although the 
supply of food may be ample (cf. Robertson and Sang, 1944). In 
some kinds of animals the ratio of males to females is also significant 
in controlling success of reproduction. Although many animals do 
best when the two sexes are present in equal numbers, a one-to-one 
ratio is detrimental for some polygamous species. Among pheasants, 
for example, each cock normally maintains a harem of about five hens, 
and, if more males are present in the population, the consequent fight- 
ing and disturbance of the incubating females greatly reduces the 
success of the breeding. With species requiring highly specialized 
places for breeding or nesting, the first effect of an enlarged popula- 
tion may be a critical shortage of breeding sites. 




Photo by Swem 

FIG. 9.5. "Deer-line" formed by denuded branches of Douglas fir and Ponderosa 
pine, caused by browsing of overabundant deer in Oregon. 



Harmful Effects 321 

Perhaps the most generally harmful effect of increasing numbers 
among animal populations is the competition for food. In the photo- 
graph, Fig. 9.5, a "deer-line" is distinctly visible on the vegetation at 
a height of about 2 m. This denudation of the trees has been caused 
by the browsing of an excessive number of deer. With the shortage 
of food the growth of the deer population will be curtailed, and, if 
all the edible vegetation throughout the area is removed, the deer 
will be in danger of starvation. In extreme instances the vegetation 
may be permanently injured with the result that the area will sub- 
sequently not support as many deer as previously. 

The effect of competition for food on the growth of bluegill sunfish 
is well demonstrated by records of farm fish ponds. In one test a 
pond was stocked in March with fish averaging 5.7 g. When weighed 
again in June of the same year, the average size of the fish had in- 
creased to 76 g. During this month each pair of sunfish produced an 
average of 4000 young. Since the young were not removed either 
artificially or by a predatory species of fish, the new generation en- 
tered into direct competition with the adults for food. When the 
pond was drained in November of the same year, the average weight 
of the parent group had decreased from 76 g to 54 g, showing that 




Swingle and Smith, 1942 

FIG. 9.6. Effect of overcrowding on the growth of bluegill sunfish. Both fish are 
1 year old: upper specimen from a pond stocked with 3750 fish per hectare (1500 
per acre); lower specimen from a pond stocked with 450,000 fish per hectare 

(180,000 per acre). 



322 Relations within the Species 

these fish had not only failed to gain but had actually lost weight 
as a result of excessive competition. Another instance of the curtail- 
ment of growth of sunfish brought about by overpopulation is shown 
in Fig. 9.6; 



Beneficial Effects 

In the previous section we have seen that, if the population of a 
species within any area continues to increase, a point will eventually 
be reached at which harmful effects are produced. Before this point 
is reached, the members of the population may receive definite bene- 
fit from the presence of others of the same species. An isolated in- 
dividual or a single pair of organisms is often not as able to deal 
successfully with the environment as a group. This subject has been 
extensively discussed by Allee (1931 and 1951). Moderate increase 
in abundance of a plant or an animal may afford protection from 
enemies or from physical features of the environment. It may ac- 
celerate reproduction and improve survival. The degree and nature 
of activity is also influenced by the density of the population ( Schuett, 
1934). Learning by fishes and other animals has been shown to be 
more rapid when other individuals are present than when they are 
isolated (Welty, 1934). Groups of the same species also make pos- 
sible a certain division of labor and the beginnings of social organi- 
zation. Some of these effects of increased numbers that are bene- 
ficial within the species will be considered in the following sections. 

Protection. Animals and plants in groups often protect each other 
against harmful features of the environment without any special 
group organization. In a thick stand of trees a higher humidity 
will be maintained and a better resistance to wind and water erosion 
will be experienced than if the same individuals were growing in a 
widely spaced manner. Furthermore, since the blowing away of 
fallen leaves is largely prevented in a dense grove, the humus and 
moisture content of the soil is increased. Groups of animals have 
been shown to be less susceptible to poisons, oxygen lack, extreme 
temperatures, and other environmental dangers than single indi- 
viduals. 

The large number of honeybees living close together in a hive 
enables these insects to modify the temperature inside their micro- 
habitat to an extent that would be utterly impossible were the bees 
living as solitary individuals. As winter conditions arrive, the bees 
are able to maintain the temperature within their cluster in the hive 
at 25 to 30C by increased muscular activity. During hot days in 



Influence on Reproduction 323 

summer the bees force air through the hive passages with their wings, 
thus increasing evaporation and lowering the inside temperature. 
Owing to this primitive "air-conditioning" temperature inside the hive 
fluctuates only slightly in spite of extreme external conditions ( Uvarov, 
1931). 

A group of animals is often protected against its predators by the 
simple effect of numbers. A flock of small birds does not attempt to 
fight off an attack by a hawk and probably could not do so; hawks 
generally obtain their prey by picking off stray individuals. Perhaps 
the hawk is less likely to be successful in catching his prey from a 
flock because of the confusion of numbers, just as a tennis player is 
distracted by having his opponent throw three balls to him at once, 
with the result that he does not succeed in catching any one of them. 

A favorable effect on survival of an optimal degree of crowding 
has been found in laboratory cultures of Drosophila and of Daphnia 
and in natural populations of the sessile rotifer, Floscularia. At popu- 
lation densities of 5 and 10 per 50 cc of culture medium Daphnia lives 
considerably longer than at densities of either 1 or 25 per culture 
unit in tests at 25C. At 18C mortality appears to be least at a 
density of about 75. Lower metabolic rate or better control of harm- 
ful bacterial contaminants by the supraminimal populations are sug- 
gested as possible explanations (Pratt, 1943). The mean length of 
life of Floscularia is practically doubled if the young rotifer, instead 
of living a solitary existence on a water plant, attaches to the tube of 
an older Floscularia and thereby establishes a small colony. No ex- 
planation of this striking fact has come to light (Edmondson, 1945). 

Influence on Reproduction. Since most animals and plants re- 
produce by sexual methods, it is obvious that too great a scarcity of 
individuals of the opposite sex will reduce the rate of breeding. In 
an earlier chapter we have called attention to the probability that 
this constitutes a serious limitation for deep sea fish and perhaps 
also for whales and other aquatic animals of low population density. 
Luminescence and underwater sound have been suggested as possibly 
serving as aids for males and females to find each other in the huge 
expanses of the ocean. The same limitation affects land animals and 
plants that are very sparsely distributed. 

A similar situation on a much smaller scale is presented by flour 
beetles dispersed in a large uniform volume of flour. Investigations 
with Tribolium confusum have shown that when only two individuals 
are present in a "microcosm" of 32 g of flour, the number of young 
produced per female is considerably less than when four individuals 
inhabit the same volume. The explanation is that fecundity is in- 



324 Relations within the Species 

creased by recopulation, and recopulation increases with the density 
of the population. At densities higher than four, however, the fact 
that the beetles eat their own eggs progressively curtails the success 
of reproduction. This cannibalism eventually sets the upper limit of 
the population (Park, 1941). In experiments with ciliates the re- 
productive rate was found to increase with the size of the initial popu- 
lation. This effect may be due to the release into the medium of a 
beneficial substance produced by the protozoans, to the low absorp- 
tion per individual of some inimical material originally present, or 
to more adequate control of the bacterial population ( Mast and Pace, 
1946; Alice, 1951). 

Another effect of numbers on success in reproduction involves re- 
actions, some of which may be considered to be in the realm of 
group psychology. In many animals breeding activity is initiated 
only when an aggregation of more than a certain minimum size has 
been formed. The necessity for the existence of a sizable group in 
order that reproductive behavior will be called forth is clearly seen 
among social insects and among certain fishes, birds, and mammals, 
as well as among other types of animals. 

This effect of numbers is particularly striking in birds that breed 
in colonies ( Darling, 1938 ) . Anyone who has visited an island where 
terns are nesting or a cliff where gannets have established a rookery 
(Fig. 1.2) will have obtained an indelible impression of the atmos- 
phere of excitement caused by the sight of thousands of wheeling and 
darting birds and by the sound of their screaming calls. The visual 
and auditory stimuli produced by this uproar are necessary, or at 
least valuable, in preparing the birds psychologically for mating as 
well as for setting off and integrating the elaborate behavior patterns 
involved in nest building, incubation, and feeding the young. If 
there are too few birds in the colony, the reproductive cycle will not 
be successfully completed. Furthermore, the large concentration of 
birds often induces synchrony in reproduction, and this provides a 
certain amount of protection since predators cannot destroy a serious 
proportion of the eggs and young if they all appear within a short 
period. Even without synchronous reproduction large colony size 
has the advantage of a relatively smaller periphery where young may 
wander away or be attacked by enemies. This need in certain species 
for group stimulation accounts for the failure of isolated pairs to breed 
and for inefficient breeding as seen, for example, in small colonies of 
gannets (Fisher and Vevers, 1944). An extreme illustration of these 
principles is the report that a minimum of 10,000 birds, nesting at a 
mean density of 3 nests per sq m, is necessary for the establishment 



Influence on Reproduction 325 

of a successful breeding colony of the guano-producing cormorant 
on islands off Peru (Hutchinson, 1950). Among mammals, popula- 
tions of muskrats smaller than about 1 pair per linear mile of stream 
or per 35 hectares of marshland appear not to breed successfully 
(Errington, 1945). 

Degree of crowding can influence the sex and structure of some 
species and the coordination of these features with the natural life 
cycle. Crowding has been shown to influence the sex ratio in certain 
cladocerans, in Bonellia, and in Crcpichda. A/own, a common clado- 
ceran inhabiting temporary ponds, reproduces parthenogenetically 
under ordinary conditions with the production chiefly of females. If 
the population becomes crowded, however, males begin to appear. 
This reaction is obviously related to the reduction of space attending 
the drying up of a pond and necessitating the production of resistant 
eggs if the population is to survive. Since resistant eggs are produced 
only sexually, males must be present in the population. The produc- 
tion of males by crowding is thus nicely attuned to the needs of these 
animals under the exigencies of life in temporary ponds. 

In the aphids, or "plant lice," which attack vegetation, the winter 
eggs hatch into wingless females; these reproduce parthenogenetically 
under ordinary circumstances, with the result that the population on 
each plant grows rapidly. If the plant host becomes overcrowded 
and consequently withers, the destruction of the whole group of 
aphids is threatened. However, crowded conditions react on the 
aphids in such a way as to bring about the production of winged 
females. This extraordinarily neat adaptation makes possible the 
migration of many of the aphids to other plants, relieving the con- 
gestion at home and establishing new centers of population growth. 
Later in the season males appear and sexual eggs are produced that 
tide the population over the winter period and complete the cycle. 

Another effect of the size of breeding population is in relation to 
genetic elasticity and, hence, the adaptability of the progeny of a 
species to varied conditions. Most species of plants and animals in 
nature are composed of a great many biotijpes, that is, types of indi- 
viduals that grow and react differently because of different genetic 
constitutions. Owing to varying environmental conditions certain 
biotype groups become established in different ecological regions of 
the range of each species. These ecological subdivisions of the 
species are known as ecotypes and are genetically distinct races. 
Since the ecotypes are interfertile, they are placed in the same taxo- 
nomic species. Ecotypes are sometimes recognized as subspecies, 
but in other instances they are not sufficiently distinct morphologically 



326 Relations within the Species 

to warrant that designation. Each ecotype is the result of selection 
by its environment and has become specially adapted for a particular 
set of conditions. Wide-ranging species are represented in different 
parts of their ranges by different ecotypes. 

A small population of an ecotype, particularly if it is isolated, will 
tend to become less variable genetically because of inbreeding; that 
is, it will come to contain fewer biotypes. With less adaptability 
the population will be less likely to survive bad conditions and will 
fail to respond quickly to the occurrence of good conditions. In con- 
trast, a larger population with more biotypes is more likely to include 
some individuals that can withstand adverse circumstances and that 
can take advantage of new variations in the environment. These 
relations probably account, in part at least, for the fact that certain 
reduced populations fail to spread widely or to recover a former 
abundance although environmental conditions appear favorable 
(Cain, 1944, Ch. 16; and Allee, 1951, Ch. 4). The isolation of seg- 
ments of a varying population also plays a major role in influencing 
the course of evolution. This large topic, which is beyond the scope 
of the present book, is considered in the works referred to above and 
also by such authors as Elton (1930), Mayr (1942), and Lack 
(1947). 

An example of the failure of a small population of an ecotype to 
spread readily is furnished by the distribution of wild irises in Canada 
(Fig. 9.7). Iris setosa has considerable morphological variation and 
is distributed widely in western Canada and Alaska. The subspecies 
Iris setosa var. canadensis, however, exhibits high morphological con- 
stancy and is limited to the Gulf of St. Lawrence region. This sub- 
species survived the ice age, during which other luxuriant types may 
have succumbed, but as a relict ecotype it emerged so uniform genet- 
ically that during the intervening centuries it has been able to repopu- 
late only the immediate area and is slow in adapting itself to other 
environments. 

The disadvantage of low numbers may also be illustrated from ex- 
perience in the oyster fishery. Around the shores of Great Britain, 
and elsewhere, oyster populations have been greatly reduced by over- 
fishing. However, after restrictions were placed on the amount of the 
oystermen's harvest, or in other areas after fishing had been aban- 
doned entirely because of unprofitable yields, the populations often 
failed to rebound to their previous large sizes. Gross and Smyth 
(1946) believe that the explanation is to be found in the lack of 
genetic flexibility in the small isolated groups of these sessile animals. 

A small population not only may be unable to grow rapidly and to 



Division of Labor 327 

spread but also, after it has been reduced below a critical size, it may 
be unable to hold its own. This latter possibility introduces the prin- 
ciple of the minimum population which states that in order for a popu- 
lation to survive indefinitely in an environment its numbers must be 
maintained above a critical minimum. 

A classic illustration of this principle is furnished by the fate of the 
heath hen, a bird that was formerly abundant in Massachusetts and 
may have been distributed from Maine to Delawo.re (Gross, 1928). 
By 1880 the heath hen was restricted to Martha's Vineyard Island, 
and a realization of its low abundance led to the establishment of a 
large reservation for the birds on the island. The heath hens in- 
creased to about 2000 in 1916, but a fire, a gale, and a hard winter 
with a great flight of goshawks decimated the population, leaving 
fewer than 50 breeding pairs. Numbers continued to decline irregu- 
larly until only 20 birds were counted in 1927, and the last bird was 
seen in 1932. In spite of elaborate protection this species could not 
be saved from extinction once the population had been reduced 
below its critical size. 

Division of Labor. Beneficial effects of increasing numbers may 
also be brought about by a division of labor made possible within 




FIG. 9.7. The discontinuous range of Iris fietosa (open circles) and its varieties: 
canadensis (small black dots) and interior (large black dots). Hatched lines in- 
dicate the maximum extent of the Pleistocene glaciations. (Cain, 1944, modified 
from Anderson, Copyright, Harper and Bros., New York.) 



328 Relations within the Species 

the group. In some sessile colonial animals, such as certain of the 
coelenterates, different individuals are specialized for definite func- 
tions. This differentiation of zooids permitting a division of labor 
within the colony reaches its highest development in siphonophores 
such as the Portuguese man-of-war. Different zooids of this animal 
are specialized for flotation, protection, nutrition, or reproduction. 
In some siphonophores zooids are also specialized for locomotion and 
produce a movement of the whole colony by rhythmic ejection of 
water from their cavities. Certain colonial Protozoa and Protophyta 
also show a division of labor. Although this development among 
unicellular colonial forms is very primitive, its advantages may have 
led in the past to the evolution of multicellular organisms. 

Among organisms that form groups but in which the individuals re- 
main separate we find other types of division of labor. The most 
fundamental and widespread of these is the differentiation of the 
species into two sexes. This division of labor has arisen in both the 
animal and plant kingdom, and interestingly enough the appearance 
of sex occurred long after plants and animals had evolved from their 
common ancestral unicellular form. We have here a remarkable case 
of parallel evolution in which the same adaptation arose indepen- 
dently on at least two occasions. 

A variation in activity among members of a group without any 
special morphological adaptation is seen in the phenomenon of social 
dominance. This is the establishment within the group of a social 
hierarchy in which an animal of higher position outfaces or drives 
away an animal of lower position ( Allee, 1951 ) . Such "peck orders" 
were first recognized in flocks of fowl, but now dominance-subordina- 
tion relations are known to exist among certain groups of fishes, 
lizards, rodents, ungulates, carnivores, and other animals (Collias, 
1944, 1952). The peck order among hens establishes the social posi- 
tion of the birds in the flock. A dominant hen has attained the right 
to peck a subordinate hen without being pecked back. When the 
social organization of a flock has become established, the group func- 
tions more smoothly and less fighting occurs, since protocol is recog- 
nized by all members for all group activities. 

Another type of division of labor in a group is leadership. Within 
a flock of goats one animal will become recognized as the leader in the 
wanderings of the group; but this animal is not necessarily the domi- 
nant individual in the sense of the term used above. The existence 
of a definite leadership was also demonstrated in a band of monkeys 
inhabiting a tropical island. One individual always led the way when 
the band moved through the aerial pathways of the treetops. When 



Division of Labor 329 

the leader was shot, the band was reported to remain in a confined 
area of the island, and did not take its usual trips to other parts of 
the island. 

Cooperation without any special morphological modifications among 
the individuals and with a minimum of organization is another ad- 
vantage often resulting from the formation of groups. When at- 
tacked, musk ox form in a circle with their horns extending outward 
and thus secure for themselves a protection that would not exist if 
each animal attempted to defend itself individually. A group of 
beavers working together can dam a stream that could not be success- 
fully dammed by animals operating singly. In a penguin colony 
some adults guard not only their own young but also the young of 
others while the remaining parents leave the area in search of food. 
Feeding cormorants are observed to maintain a rough line as they 
swim and dive and thus presumably improve the effectiveness of their 
fishing (Bartholomew, 1942). 

A somewhat more elaborate division of activity within the group 
is found in certain species during the breeding season. In sea lions 
(Fig. 9.8), fur seals, elk (Fig. 9.9), deer, and other mammals, as well 
as various game birds, the population of a region becomes organized 
into breeding units each consisting of a male, his harem, and sub- 
sequently their young. After the mating season the harem groups are 
broken down, and the animals may reorganize themselves into sepa- 
rate bands of males, females, and young for the remainder of the year. 
Group organization in the fur seals (Callorhinus ursinus) may be 
taken as an example. The bulls arrive first early in May at the 
breeding grounds on the Pribilof Islands, and each stakes out a 
breeding site for himself which he jealously guards. When the cows 
arrive in mid-July, each of the larger and more senior of the bulls 
collects a harem for himself within his own territory. The remain- 
ing bulls, kept away by ferocious fighting, form a group of disap- 
pointed bachelors. Bulls with harems are continually being chal- 
lenged by the bachelors, and, although each established bull drives 
off the intruders for one or more seasons, eventually each bull ages 
and after losing a fight or a series of fights he is forced to relinquish 
his harem to a new master. Soon after the females are organized 
into harems the pups resulting from the previous breeding season are 
born, and subsequently mating with the bulls takes place. When 
breeding has been completed, the harems disintegrate. After a 
period of feeding and of nursing the pups, the seals reorganize them- 
selves in groups consisting of males only, and other groups made up 
of females and pups. In these groups the seals leave the Pribilofs 



330 



Relations within the Species 



.. 



'",.. ,. ;*% ^iit?^ V T 



Photo Alfred M. Bailey, National Audubon Society 

FIG, 9.8. Breeding groups of sea lions on San Benitos Island off the west coast of 
Mexico. Each bull sits erect guarding his territory and presiding over his harem 

and pups. 




fhoto U. V. National I'arK Service 



FIG. 9.9. Bull elk and his harem in Rocky Mountain National Park in late 

September, 



Division of Labor 331 

in October and travel to their winter quarters along the California 
coast (Allen, 1870). With this social organization in operation 
among the fur seals, the non-breeding males can be killed for their 
pelts without reducing the productivity of the herd; hunting is re- 
stricted in this way by international agreement. Similar organization 
of the herd into harems during the breeding season and into larger 
non-family groups during other parts of the year is found among the 
European red deer, Cervus elaphus (Darling, 1937) and among the 
American elk or wapiti, Cervus canadensis (Murie, 1951). 

The most complex types of division of labor are exhibited by man 
himself and by certain insects. The division of labor in man is a 
learned behavior and occurs without structural modification. In com- 
plete contrast, the behavior exhibited by members of an insect colony 
is instinctive and the individuals performing various functions in the 
colony are specialized morphologically and physiologically (Fig. 
9.10). This differentiation of members of the same species is devel- 
oped to its greatest extent among the termites and ants (Wilson, 
1953). Since the division of labor in an insect colony is accompanied 
by structural specializations and instinctive behavior, it is less flexible 
than that occurring in mammalian groups, and especially in man's so- 
ciety. This fact and their size limitations have no doubt largely pre- 
vented the insects from attaining a more dominant position in the 
world. 

Among groups of animals all degrees of integration exist from 
aggregations with no organization to highly elaborated societies. 
Allee (1951) has developed the theory that societies have evolved 
from aggregations. If the existence of a group confers definite sur- 
vival value on the individuals, the group will tend to persist. Those 
reactions that lead to the formation of the group will also persist, 
and these will constitute the beginning of social behavior. The co- 
operation that we now observe among the individuals may have 
resulted as a consequence of the formation of the group. The 
division of labor appeared as chance variation and persisted because 
of its survival value; it is not necessarily conscious nor purposeful. 

According to the foregoing ideas the elaborate social organization 
found among a few species may therefore have evolved through the 
following steps. First, others of the same species are tolerated in a 
restricted space and then definite reactions to their presence are de- 
veloped. If survival values result either through behavior or physio- 
logical adjustments, a tendency for an increase in the permanency of 
the group will exist. Following this, a further development of the or- 



332 Relations within the Species 

ganization of the group may take place with the appearance of lead- 
ership, dominance, and/or the division of labor. It is impossible to 
say just when an aggregation becomes sufficiently organized to be 
termed a society all gradations exist. The evidence is strong, how- 
ever, that the beneficial effects of increased numbers have played a 
considerable part in the development of organized groups and fully 
differentiated societies. 




FIG. 9.10. The various forms of the termite Kalotermes fiavicollis and their de- 
velopmental origins. The eggs (bottom) hatch into young nymphs which after 
5 to 7 molts reach the pseudergate stage (individual in center). From this stage 
the termite can change into a winged reproductive (top) by way of two wing- 
padded nymphs. At intermediate stages environmental influences may cause the 
nymphs to change into supplementary reproductives (left) or, by way of soldier 
nymphs, into soldiers ( right ) . Most of the nymphs do not differentiate and these 
function as workers. All these stages are present in the termite colony, and their 
activities are integrated in its maintenance. (Liischer, 1953, Set. American, draw- 
ing by E. Mose. ) 



Natality and Mortality 333 



POPULATION DEVELOPMENT 

The organisms inhabiting an area at a given time constitute a 
population. If the organisms all belong to one species, they form a 
single-species population. In most natural situations several to many 
kinds of plants and animals coexist in the same habitat. The in- 
habitants may then be regarded as composing a corresponding num- 
ber of single-species populations that are intermingled, or alternatively, 
as forming one mixed or multi-species population. The interspecific 
relations of mixed populations will be treated in the next chapter 
Here we shall discuss the quantitative relations that arise during the 
growth and fluctuation of single-species populations under the in- 
fluence of the harmful and beneficial interactions considered in the 
foregoing sections. 

Principles governing population dynamics are found to apply 
equally to special situations, such as laboratory cultures, in which 
only one species is present, and to natural areas in which many popu- 
lations exist together. The presence of other species acts as part of 
the environment in influencing the changes in the population of the 
species under consideration. 

Natality and Mortality 

The abundance of a species in an area tends to increase because of 
reproduction, and to decrease because of death. The rate of repro- 
duction depends upon the birth rate or natality, and the rate of death 
is referred to as mortality. The dispersion of members of the species 
also affects abundance positively in the case of immigration, and 
negatively in the case of emigration. We shall consider first the sim- 
plest situation in which no dispersion is taking place, leaving a discus- 
sion of the influence of migration into or out of the area to a later 
section. The maximum possible rate of reproduction for a given 
species under optimal conditions is termed the potential natality. In 
natural situations the potential natality is rarely, if ever, attained be- 
cause the birth rate is inevitably reduced by one adverse circumstance 
or another. The actual birth rate under the existing conditions is 
referred to as the realized natality. In parallel fashion the lowest pos- 
sible death rate for a given species in the best of circumstances is the 
potential mortality, and the actual death rate is the realized mortality. 

Natality and mortality vary not only from species to species but 
also according to the age of the individuals. Natality is usually 



334 Relations within the Species 

highest during the middle of the life span after the individual has 
become mature and before it has become senile. Variation of mor- 
tality with age or life stage differs greatly among different kinds of 
animals and plants. In some species extremely high mortality is ex- 
perienced in the egg, larval, or seed stages; in others high mortality 
does not occur until late in life. 

The potential natality of every species of plant and animal is greater 
than its potential mortality, and hence, under favorable conditions, 
every species always has the capacity to increase. If under existing 
conditions the realized natality also is greater than the realized mor- 
tality, the population will actually increase. If the two rates are 
equal, the population will be stationary; but, if the realized mortality 
is greater, numbers will diminish. A birth-death ratio defined as 

100 .. ir , is known as the vital index. 
deaths 

Biotic Potential and Environmental Resistance. The maximum 
possible rate of increase (highest vital index) for a population of a 
species occurs under ideal conditions in which the birth rate is the 
highest possible for that species (potential natality) and the death 
rate is the lowest (potential mortality). Maximum birth rate is de- 
termined by the largest number of viable progeny (spores, eggs, 
young, or seeds) that an animal or a plant can produce and the 
frequency of reproduction. Minimum death rate is determined by 
internal factors controlling survival when environmental factors are 
all completely favorable. The values of maximum birth rate and 
minimum death rate are thus fixed by life processes inherent within 
the organism, and the maximum rate of population increase, or the 
biotic potential, is an innate characteristic of each species. The 
value of the biotic potential, or potential increase as it is sometimes 
called, differs widely from species to species; contrasting examples of 
differing rates were given in Chapter 1. As will be more fully dis- 
cussed subsequently, evolutionary processes have established certain 
relations between the biotic potential of each species and the exigen- 
cies of its existence. 

Under natural conditions the full biotic potential of an animal or a 
plant population is ordinarily not realized since conditions are rarely 
completely favorable, Harmful climatic changes, attacks by preda- 
tors and diseases, and other external circumstances curtail the growth 
of the population. As the population grows, the increase in numbers 
itself produces changed conditions. A moderate increase in density 
may sometimes have an ameliorating effect, as we have seen, but 
sooner or later the detrimental effects of overpopulationscarcity of 



Biotic Potential and Environmental Resistance 335 

food supply or of breeding sites, accumulation of metabolites, and the 
like will appear. The combined effect of these factors tending to 
curtail population growth is called environmental resistance. The 
capacity to increase resides within the species, but the degree to 
which it is realized is determined by the environment, including the 
changes in the environment brought about by the species itself, and 
aspects of the environment consisting of other members of the species. 
Thus the actual rate of the increase of a population is determined by 
the balance struck between biotic potential and environmental 
resistance, 

Certain features of the environment are largely or entirely unaf- 
fected by changes in the density of the population. These are density- 
independent factors. For example, an increase in the abundance of 
a species in a marine area does not affect the temperature or the 
salinity of the water, but changes in these factors, harmful or other- 
wise, are brought about by agents unrelated to the density of the 
population. Other changes in the environment are directly related 
to the abundance of the animals or plants concerned; these are 
density-dependent factors. Scarcity of such necessities as food, oxy- 
gen, or breeding sites may become increasingly acute as a population 
of animals grows; lack of nutrients, excessive pH values, or other 
inimical condition may be brought about by the growth of a plant 
population. In addition, the susceptibility of organisms to disease 
as well as the ease of transmission is often increased as the density of 
the population grows. Generally speaking, the physical features of 
the environment tend to be density-independent and the biotic in- 
fluences are often density-dependent factors, but the reverse is some- 
times true, and certain factors may change from one category to the 
other according to circumstances. For example, the predation of a 
tawny owl ( St rix aluco ) on a population of mice in an English wood- 
land was found to be a density-independent factor since the owl was 
observed to eat only 4 to 6 wood mice (Apodemus sylvaticus) each 
day, regardless of the size of the prey population (Miller, 1951), But 
in other situations predation will be a density-dependent factor if 
each predator kills more prey when the prey are abundant. During 
periods of plenty many predators are known to kill more than they can 
eat. Increased density of prey may also cause increase in the pred- 
ator factor by making possible increased reproduction and growth of 
the predator population as well as by attracting predators from neigh- 
boring areas. 

The question just discussed of whether the density of a population 
controls or modifies a certain environmental factor must be carefullv 



336 Relations within the Species 

distinguished from the question of whether the influence of the factor 
on the population is related to the size of the population. For ex- 
ample, the winter temperature in most habitats is a density-independ- 
ent factor since its value is not affected by the number of organisms 
present, but the effect which the temperature has may or may not be 
related to the size of a population of animals. In some habitats ex- 
treme cold will kill the same percentage of a large population as of 
a small one, with the result that the number of animals succumbing 
is proportional to abundance. In other instances the habitat will 
provide adequate shelter (from extreme temperature or other danger) 
for a small number of animals but inadequate shelter for a large num- 
ber. As a consequence a much greater fraction of the large popula- 
tion would be harmed than of the small population. If in another 
situation a factor killed off a constant number of animals in an area, 
it would have a greater relative effect on a small population than on 
a large one. 

Form of Population Growth 

When plants or animals reproduce, they add more individuals to 
the population, and the enlarged breeding stock then has the capacity 
to produce a still larger number of progeny. Thus the population 
tends to grow at an ever-accelerating rate. Sooner or later, however, 
harmful density-dependent influences begin to take effect. If we 
assume for a moment that no other interfering factors are present, the 
growth of the population will follow a mathematically prescribed 
form. 

Let us take a simple numerical example. Suppose we assume that 
under the most favorable conditions a certain pair of animals can pro- 
duce 6 young during a year and that the resulting population suffers 
a mortality of 2 during the year. We then have: 

N + A - M = tf , 
S + (5 - 2 = 6 



in which N and N, represent the populations, at the beginning and 
at the end of the year, respectively, A is augmentation, M is mortality, 
and R is the biotic potential or rate of potential increase per genera- 
tion. The generation time in this example is one year. At the be- 
ginning of the second year our sample population stands at 6, and, 
since there are more individuals to breed, a larger number of progeny 



Logistic Curve 337 

will be produced during the second year than during the first. As- 
suming the same rates of reproduction and mortality, 18 young will 
be produced, 6 will die, and the total population at the end of the 
second year will be: 

Ni + A - M = N* 

6+ 18 - 6 = 18 
or 

RN l - N, 
3 X 6 = 18 

If the same rate of increase continues unimpeded, the population will 
have grown to 54 at the end of the third year, 162 at the end of the 
fourth year, and so on, as shown in Table 18. The population, which 
under these circumstances is exhibiting a geometric or "logarithmic" 
increase, is represented by the equation: 

N = #' 

where f in years starts at 0, This is indicated graphically by curve 
(A) Fig. 9.11. The rate of change of population size is given by the 
equation : 

dN A/ , 7) 
^ = N loge R 

which means that growth rate is proportional to size of population. 

TABLE 18 

GROWTH OF HYPOTHETICAL POPULATIONS IN WHICH INCREASE Is 
(1) UNIMPEDED AND (2) SELF-LIMITED 

Unimpeded Increase 

Years 01234 5 6 7 

Total population 26 18 54 102 486 1458 4374 

Numbers added 4 12 3(> 108 324 972 291(5 

Self-limited Increase 

Years 0123 4 5 6 7 

Total population 26 14 27 39 46 48 49 

Numbers added 4 8 13 12 7 2 1 

Logistic Curve. Since the full biotic potential of a species is not 
realized under most natural conditions, the population does not in 
fact increase as fast as it could if its growth were completely unim- 
peded. However, if the natality rate remains above the mortality rate, 
the population will continue to grow and will increase at an accelerat- 
ing rate. Nevertheless, the increase in the population will eventually 
produce conditions harmful to itselfdensity-dependent factors will 
come into play. The rate of growth will then be progressively cur- 
tailed until it reaches zero when the population reaches the largest 
size possible for it within the area concerned. If the harmful effect 



338 



Relations within the Species 



of crowding increases proportionally, the rate of change of population 
size can be expressed as follows: 



dN _ (K - N 
dt V K 



N log, R 



where R is the hiotic potential per generation, N is the size of the popu- 
lation at any moment, and K is the ultimate maximal size possible for 
the population in the given area. In words, the equation simply says 
that the rate of increase of the population is equal to the potential 



80 
70 
60 
50 
40 
30 
20 
10 


20 

10 



LA 




"-Inflection point 



Asymptote 




1234 
Years 

FIG. 9.11. Upper figure: graphs showing theoretical increase in total numbers 
during (A) unimpeded and (B) self-limited population growth (logistic curve). 
Lower figure: graph of numbers added to population in each time interval during 
the logistic growth shown by (B) above (values plotted at midpoint Of each time 

interval). 

* For further discussion of these relations reference may be made to H. C. 
Andrewartha and L. C. Birch. "The Distribution and Abundance of Animals" 
1954, Univ, of Chicago Press. 



Logistic Curve 339 

increase limited by the degree of realization of maximal size. When 
the growth of a self-limiting population is expressed graphically, a typi- 
cal S-shaped curve is obtained known as the logistic curve, shown as 
(B) in Fig. 9.11. The curve is truly logarithmic only at its begin- 
ning, departs from the logarithmic increase as impeding factors be- 
come effective, reaches an inflection point at which the acceleration of 
growth becomes negative, and approaches an asymptote representing 
the limiting size (K) of the population. 

If we now apply to our previous numerical example the more real- 
istic condition that increase is progressively curtailed as the popula- 
tion grows, we obtain the growth values, rounded to integers, indicated 
in the lower part of Table 18 with a maximal population size in this 
case assumed as 50. Besides the population totals at the end of each 
year (or other period) we are also interested in knowing the numbers 
added to the population during each unit of time. With unim- 
peded growth the annual increment becomes larger and larger in- 
definitely; but with self-limited growth the annual increment passes 
through a maximum at the time when the logistic curve reaches the 
inflection point. In our example the numbers added per year in- 
crease to a maximum of 13 during the third year and then drop off 
nearly to zero in the seventh year. A curve showing these changes in 
the increments to the population and their relation to the logistic curve 
is presented in the lower portion of Fig. 9.11. The fact that the 
population has approached its asymptote with annual increment ap- 
proaching zero does not necessarily mean that little or no reproduction 
is taking place in the populationit simply means that births are com- 
pletely offset by deaths, natality is equaled by mortality, or A = M . 
Under these circumstances reproduction may continue to be high ac- 
companied by high mortality, or reproduction may be low with low 
mortality; as long as A and M are equal the size of the population will 
not change. 

Early in the history of the population only small numbers are added 
each year because the breeding stock is small. During the middle 
period annual increments are large, but, as the population reaches its 
maximum size, small annual increments again occur either because 
breeding is sharply curtailed or because the young produced suffer 
severe mortality. The reader should particularly notice that the 
largest annual increment is not found when the population is at its 
maximum, but occurs at the inflection point of the logistic curve, that 
is, at the time when the population is growing most rapidly. 

Populations of a wide variety of organisms, ranging from bacteria 
to whales, have been found to follow the logistic curve in their growth 



340 Relations within the Species 

form. Illustrations of such curves of growth for laboratory cultures 
of Protozoa, yeasts, Drosophila, flour beetles, and water fleas, and for 
natural populations of bees, ants, thrips, sheep, and other animals are 
discussed in further detail by Allee et al. ( 1949, Ch. 21 ). The growth 
of man's population follows a similar pattern whether examined in 
individual regions or in the world as a whole. A plot of the census 
records for the United States for the years up to 1940 is shown in Fig. 
9.12, and the curve has been extrapolated by fitting the logistic func- 



2UU 
175 


















,'' 


,* ' 


* 










150 














/ 


^/ 

^ 
















.1 










/< 


/ 

r 




















c 
ifiO 






i 


^ 

# 
^ 
























1. 7S 






/ 


























" 
"in 




y 




























?s 


/ 































































Year 

FIG. 9.12, Growth curve of the population of the United States, showing census 
counts from 1790 to 1950. The logistic function has been fitted to the counts 
from 1790 to 1910 and extrapolated to 2100. The agreement of the extrapolation 
with the counts for 1920 to 1950 is shown, and a cessation of growth about the 
year 2100 is indicated. (Modified from Pearl, Heed, and Kish, 1940.) 

tion. The agreement of the 1950 census figure of 151 million with 
the extrapolated curve and the indication of an asymptote at about 
184 million in the year 2100 may be observed. 



Equilibrium and Fluctuation 

The logistic curve discussed in the previous section applies only to 
periods of population growth (when A>M) and to situations in 
which the rate of increase is controlled only by density-dependent 
factors. Since the inhabitants of a natural area have mostly been 
present for a long time, we see the initial stages of population growth 
only in special instances. The early part of population increase is 



Equilibrium and Fluctuation 341 

observed when a bare area is invaded, when a new species is intro- 
duced, or when a check holding a species to a low number is sud- 
denly removed. Aside from these special situations the changes ob- 
served in population size involve the action of both density-dependent 
and density-independent factors usually long after the original growth 
of the population. In both laboratory cultures and in natural habi- 
tats, after the initial attainment of maximal size, a population will 
(1) maintain itself at about the same level for a long period, (2) 
decline and eventually become extinct, or (3) fluctuate regularly or 
irregularly. 

If the population approaches its asymptote in such a way that the 
supply of food, and other necessities, and the removal of harmful by- 
products keeps pace with growth, then the population will maintain 
itself at or near this equilibrium level (A M) until outside condi- 
tions are altered. Under these circumstances the reproductive rate 
may be high or low, but, as long as it is exactly offset by mortality, 
the population size will not change. Nevertheless the magnitude of 
A and M may exert an important ecological effect on evolution. If 
A is very large, as is true of many invertebrate animals, the accom- 
panying high mortality will generally take place when the progeny 
are young and usually before reproduction has occurred. On the 
other hand, if A and M are small, a much larger proportion of the 
young animals may live long enough to reproduce before they are 
eliminated from the population. In the former circumstance, most 
of the mutations appearing in the population will be lost, but in the 
latter case a larger percentage will be retained long enough to affect 
the next generation. We find that the possible effect of this difference 
corresponds to the generally more rapid evolution of mammals, for 
example, than that of invertebrates. 

In other populations the harmful conditions that were produced by 
increasing numbers and that brought the growth of the population to 
a stop may progressively intensify. Under these circumstances the 
greater and greater scarcity of food, the accumulation of metabolites, 
or other inimical change will cause a decline in the population either 
immediately or after a period of time. If the changes in the environ- 
ment brought about by overcrowding are irreversible, extinction of 
the population will eventually follow. This result is commonly seen 
in laboratory cultures of bacteria, Protozoa, and other organisms. In 
nature a similar fate may overtake a population developing on a small 
island as a result of the introduction of breeding stock by natural 
processes of dispersal or by man. Rats, goats, rabbits, or other ani- 
mals, escaped from explorers' ships or introduced by colonists, often 



342 Relations within the Species 

find conditions suitable for rapid growth and reproduction. Unim- 
peded, geometric increase in the population ensues until all suitable 
food or other necessity on the island is exhausted. Thereupon starva- 
tion, and often disease, decimate the population, and the death of all, 
or nearly all, of the animals quickly follows. 

More usual than either of the foregoing is the third type of situa- 
tion in which the population overshoots its equilibrium level, but, 
after a reduction in numbers, conditions are ameliorated sufficiently 
for the population to increase again. The repetition of this process 
causes fluctuations. If these are small, the population curve is desig- 
nated as flat; if the amplitude is large and regular, the curve is termed 
cyclic, and, if irregular, especially with sudden periods of great in- 
crease, it is called irruptive (Fig. 9.13). Some ecologists refer to 



Irruptive 
Cyclic 

* '> 




Extinction 



Time 
FIG. 9.13. Diagram of types of fluctuations in populations. 

regular changes in abundance as oscillations and to irregular changes 
as fluctuations, but the more specific terminology given above will be 
employed here. The reader should understand that all gradations ex- 
ist between these types of population change. 

Fluctuations after a population has approached its equilibrium level 
may be caused in whole or in part by changes in physical features of 
the environment or in biotic influences such as the abundance of 
predators, diseases, or food organisms. These population changes 
due to the interaction of different species will be considered in detail 
in later chapters. At this time we wish to emphasize the fact that 
conditions within the population of a species can themselves be re- 



Equilibrium and Fluctuation 



343 



sponsible to a greater or lesser degree for the fluctuation in addition 
to any disturbing influences that may come from the outside, The 
causes of certain fluctuations can be traced to the reciprocating ef- 
fects of natality and mortality within the population itself. 

A neat illustration of the mechanisms involved in self-induced fluc- 
tuations of this sort is found in experiments with laboratory cultures 
of Daphnia magna (Pratt, 1943). Single females were placed in 
bottles containing 40 ml of water, and as they and their progeny re- 
produced parthenogenetically the population in each bottle grew 
in accordance with the logistic function. The culture medium was 
renewed each day, insuring ample food supply and removal of me- 
tabolites, and other external conditions were kept uniform. Follow- 
ing the initial attainment of maximum size, the population sank to a 
low ebb and then continued to fluctuate in extreme fashion in spite of 
constant environmental conditions (Fig. 9.14). The explanation is 



100 



I 80 



o 60 

I 
E 40 



20 



A 




A 



10 



20 



30 40 



70 



80 



50 60 
Time in days 

FIG. 9.14. Changes in the abundance (solid line) of Daphnia magna in a labora- 
tory culture. The variations in number of births (--) and of deaths ( ) 

that underlie the fluctuations of the population are indicated. (Pratt, 1943.) 

seen in the changes in the birth and death curves indicated in the 
diagram. Although the number of births declined after the tenth 
day, young Daphnia continued to be produced for 18 days with the 
result that more young were added to the population than could sur- 
vive when they reached adult condition. As overcrowding continued, 
the death rate rose rapidly and the birth rate dropped to zero. Even 
after the size of the population was considerably reduced, no repro- 
duction took place at first and in fact no new young animals appeared 
until the fortieth day. Animals born in adversity thus did not pro- 
duce young immediately upon reaching maturity, and the reduction 
in their reproductive capacity persisted for a long period after fav- 
orable conditions had been restored. As a consequence the popula- 



344 Relations within the Species 

tion continued to overshoot and undershoot a possible equilibrium 
level. When this experiment was repeated at a lower temperature, 
the population changed more slowly, with the result that after one or 
two oscillations an equilibrium value was reached and maintained. 
Delay in the manifestation of excessive natality and prolongation of 
the effects of overcrowding similarly act on natural populations and 
add their influence to that of external factors in causing fluctuations. 

The relative abundance of individuals of various ages in the popu- 
lation is known as the age distribution of the population. Differences 
in age distribution depend upon the species and whether the popula- 
tion is changing in size or stationary (Petrides, 1950). A rapidly 
growing population usually contains an especially large number of 
young individuals, whereas a declining population includes a rela- 
tively high proportion of old individuals. In a stationary population 
the distribution of ages is more uniform and tends to approach a 
stable pattern. Since natality and mortality vary with the age of the 
individuals, the age distribution of the population influences the birth 
and death rates for the population as a whole. Species that produce 
large numbers of young generally suffer high mortality during the 
young stages. A complete description of the mortality of a popula- 
tion is furnished by a life table such as has been constructed for man 
and has long been used by life insurance companies. Life tables and 
survivorship curves have now been worked out for a number of 
natural populations and show characteristic differences in mortality 
patterns among various species (Deevey, 1947 and 1950). For ex- 
ample, mortality in the oyster is extremely high during the larval stage 
and becomes much lower later in life, in hydra mortality is nearly 
constant at all ages, and in man mortality tends to be low for a long 
period during youth and to become high rather abruptly in old age. 

Optimal Yield 

The foregoing analysis of population development is of interest 
not only in relation to theoretical considerations but also in connec- 
tion with practical applications. In the exploitation of natural popu- 
lationseither plant or animal a harvest is desired of the largest 
number of individuals per unit of time that is possible without per- 
manently impairing the breeding stock. In other words, we wish to as- 
certain the size of the largest sustained yield that can be obtained and 
learn at what level the population should be maintained to produce 
this yield. An answer to this question is the formation of the theory 
of the optimal yield. The basic idea underlying this theory is that in 



Optimal yield 345 

the simplest situation with no modifying conditions the population 
should be maintained at the inflection point of its growth curve since 
at that point the largest increment is being added per unit of time 
and the taking of this increment would represent the largest harvest 
possible without inroad upon the breeding stock. We shall return 
later to a discussion of special circumstances applying to particular 
species that modify this basic relation and cause a different density 
level of the population to be more favorable for the practical exploita- 
tion of these species. 

To illustrate the basic principle let us imagine a flock of geese from 
which we wish to obtain as many birds as possible for food. Suppose 
that the flock originated from the establishment of one pair in a lim- 
ited area and that the growth of the population followed the logistic 
curve (Fig. 9.11) with values similar to those indicated in the lower 
part of Table 18. The largest single yield would obviously be ob- 
tained by allowing the flock to grow for 5 or 6 years and then shoot- 
ing all or most of the birds. For a sustained harvest, however, only 
the annual increment to the population should be taken; in this ex- 
ample, 13 birds could be taken every year if the flock were allowed 
to grow to 27 before harvesting. If a hunter considered exploiting 
this flock without knowing its existing si/e and found that only 7 or 8 
birds were added to the population each year, he would have to de- 
termine whether the low growth rate was due to the population being 
smaller or larger than the optimum. Low yield in any such popula- 
tion may thus be caused by overexploitation resulting in too small a 
breeding stock, or in underexpolitation resulting in harmful crowding 
of the breeding stock. The best conservation procedure often calls 
for the reduction in size of a natural population. Recognizing this 
fact, rangers now regularly cut back the populations of deer in national 
parks whenever numbers become too large because of the absence of 
wolves, panthers, or other natural enemies that otherwise would keep 
the deer in check. Consequently, in exploiting a population, it is de- 
sirable to ascertain whether density is above or below the level at 
which increase is most rapid and, if possible, to adjust the size of the 
population accordingly. 

As already suggested, several important modifying considerations 
must be added to the underlying idea of the theory of optimal yield. 
Up to this point we have discussed populations in terms of numbers 
of individuals. Fishermen do not measure their catch by counting 
the fish, but by weighing them; foresters evaluate their timber by its 
volume, not by the number of trees. Thus, the growth in size of the 
individuals must be taken into account as well as the increase in num- 



346 Relations within the Species 

bers. For such species the growth of the population is measured in 
terms of weight or volume, and, when plotted in corresponding units, 
again follows a logistic curve, although the numerical values will, of 
course, be different. Accordingly, in attempting to ascertain the 
optimal tonnage of fish to catch we must consider the growth rate of 
the population in terms of increase in weight rather than numbers. 
If there are no further modifying considerations in respect to a par- 
ticular fishery, the optimal catch may be obtained when the popula- 
tion is maintained at the level at which the largest weight increment 
will be added per unit of time, this increment being due both to an 
increase in numbers and to the growth of the individuals (Fig. 9.15). 

Other practical considerations are the availability of the population 
for exploitation and its suitability for the market. If game in a forest 
or fish in a pond are scarce, more time, effort, and expense will be re- 
quired to harvest the same number than if the population were dense. 
With sparse populations the yield per unit effort is lower, In specific 
instances, it may be better to allow the population to increase to a 
somewhat greater density than that which represents the level of most 
rapid growth in order to increase availability. Often minimum limits 
and sometimes maximum limits of size exist in relation to market- 
ability. Accordingly, the size composition of the catch must be con- 
sidered as well as its weight or volume. 

Two additional considerations that also may modify the basic plan 
for obtaining the optimal yield in respect to certain species are: the 
number of breeders necessary to provide sufficient young, and the 
rate of natural mortality. In species with a low natality such as most 
large mammals and birds, the recruitment, or number of young an- 
nually added to the population, is closely dependent upon the size 
of the breeding stock. For each new young whale there must be one 
adult female; but one fish, one oyster, or one termite can produce mil- 
lions or billions of young in a season. The spat released by only a 
few clams would provide an ample set for a large area, and, if al- 
lowed to grow up, the young produced in one year could repopulate 
a whole bay under completely favorable conditions. In such species 
the recruitment usually depends more on the environmental condi- 
tions during the early life of the new generation than on the number 
of breeders, and the optimal level at which the population should be 
maintained is determined by other considerations. 

We have seen that allowing populations to build up by avoiding 
overexploitation sometimes has advantages in the greater size of in- 
dividuals, the more concentrated populations, and the larger number 
of breeders. On the other hand, allowing too great development of 



Optimal Yield 



347 







1 1 



C+-4 O 

o __ 





03 o 3 



1 a 

i S 
. 2 

! 

!^ 

!fr 



0-3 



348 Relations within the Species 

the population will result in the disadvantages of overcrowding. 
Another consideration is the fact that the longer an animal or a plant 
remains unharvested, the longer it will be exposed to environmental 
dangers and the greater the possibility that it will die from other 
causes or be removed by predators other than man. If natural mor- 
tality is very high, the crop may have to be harvested at an earlier 
age than would be necessary should the reverse be true. The optimal 
level for the population thus varies widely according to the ecological 
relations of each species. The optimal yield is obtained when the 
rates of reproduction, growth, and mortality are so adjusted as to pro- 
duce the greatest annual increment with due regard to availability and 
other modifying considerations. More detailed discussion of both 
the theoretical and the practical aspects of this important topic is 
available in Russell (1942), Trippensee (1954), and the publications 
of the United States Department of Agriculture. 

SPATIAL RELATIONS OF POPULATIONS 
Space Requirements 

As a population grows in numerical strength and as its members in- 
crease in size, the individuals tend to come closer and closer together 
as long as the population occupies the same area. In many instances 
the habitable area is sharply limited, as it is for aquatic populations 
developing in a pond, or for land organisms multiplying on an island. 
Although in other instances populations can spread out at their mar- 
gins for a time as they develop, eventually the unoccupied area is 
completely taken up and a condition of overcrowding begins to ap- 
pear. 

Every organism requires a minimum amount of space within which 
it can carry on its necessary exchange with the external world. This 
minimum space must be sufficient to provide food and other necessi- 
ties, to absorb metabolites, and to permit reproduction. A relation 
obviously exists between the size of the organism and the minimum 
size of the space that it can inhabit. Whales do not live in ponds. 

The space required by some organisms is quite small and may be 
no larger, in one plane, than the actual dimensions of the body. The 
number of adult barnacles that can inhabit a rock, for example, is 
determined simply by the number of barnacle bases for which the 
area of the rock affords room. Barnacles can grow with their sides 
closely appressed to their neighbors because water currents bring 
food particles and oxygen and take away metabolites and progeny. 



Space Requirements 349 

Most species require more space than their physical dimensions. 
The actual amount of room needed varies greatly from species to 
species, but frequently the space required is very large either because 
some necessity is sparse in its distribution or because of certain, often 
poorly understood, psychological relations. A census of the fishes in- 
habiting the shore zone of Morris Cove, New Haven, Conn, revealed 
the presence of representatives of 32 species, but few specimens 
longer than 100 mm were found. Although the adults of small species 
were taken, only the young of the large species were present. The 
depth of water was ample for more sizable fish to move about in, but 
fish larger than the critical size were definitely excluded from this 
habitat regardless of age although the mode of action of the space 
requirement in this instance is not known (Warfel and Merriman, 
1944). 

The manifestations of the space requirement may also occur in re- 
lation to breeding, hostility, or other reaction. Insects such as the 
Canna Leaf Roller (Calpodes ethlius) have been found to breed sat- 
isfactorily under laboratory conditions if placed in containers of cer- 
tain size. They will not breed in smaller containers, nor in much 
larger confined areas such as greenhouses, which appear to be other- 
wise suitable. Cannibalism among insects often occurs when they 
are crowded together, whereas members of the same species do not 
attack each other when they meet at other times under uncrowded 
conditions. The English sparrow, a partially domesticated bird, will 
breed regularly in large aviaries but will not breed in small cages. 
The foregoing examples suffice to illustrate the point that definite 
space requirements exist not only in relation to the physical needs of 
the organism but also in relation to reactions essential in their life 
cycles. 

For sessile forms, such as the majority of plants and non-motile 
animals, the provision of minimum space is automatic to some extent. 
As crowding continues to increase, competition brings about the 
stunting and eventually the death of a portion of the population. 
This process can be readily seen in the development of a stand of trees 
from a bed of seedlings. Root competition, shading, and diseases 
gradually thin out the population. Young white pines in New Eng- 
land, for example, sprout from seeds to form a bed with a density of 
perhaps 75,000 seedlings per hectare (30,000 per acre). When these 
pines have attained an age of 60 to 80 years, they will form a mature 
stand in which the density has been reduced by natural processes to 
about 750 trees per hectare (300 per acre), or even less, without any 
treatment by the forester. Similar examples may be found among 



350 Relations within the Species 

oysters, mussels, and other sessile animals growing on substrata of 
limited dimensions; a few individuals, growing faster than the rest, 
gain the upper hand and smother their more retarded brothers. This 
automatic reduction of the population by the growth of its own mem- 
bers produces at least the minimum spacing necessary for survival, 
but it usually does not provide the most favorable spacing for the best 
development of the individuals. Man can often improve growing 
conditions for particular species in which he has a special interest by 
further thinning and this is done regularly, of course, by professional 
growers, such as oystermen, farmers, and foresters. 

For motile animals the danger exists that during favorable condi- 
tions the members of a species inhabiting a region will move too close 
together, with the result that wholesale destruction will take place 
when conditions become unfavorable again. In some species we find 
an instinctive reaction causing the animals to space themselves out to 
a certain extent at least; and the existence of this response helps to 
avoid overcrowding, as will be discussed in the ensuing section. 

Home Range and Territory 

Many kinds of animals are known to establish a center of opera- 
tions for themselves and to confine their roamings within certain 
boundaries. The area within which an animal tends to stay is known 
as its home range. Sometimes other members of the same species 
are not allowed to enter the area, or a portion of it, and any trespassers 
are forcibly driven out. The inner sanctum that is actively defended 
is called the animal's territory. The territory may be established by a 
social group, such as a hive of bees or a colony of ants, by a breeding 
pair, or sometimes by a single individual. The existence of territories 
with "exclusive rights" was recognized in an old Chinese proverb 
that states: "One hill cannot shelter two tigers." A dog's own yard 
is his territory, and he drives other dogs out with a self-assurance that 
is familiar to all of us. Within their own territories animals typically 
acquire a heightened position of dominance in relation to other mem- 
bers of the same species. 

The concept of territory was first delineated for birds. It was ob- 
served that in many species each male established a territory at the 
beginning of the breeding season. The male then isolates himself, 
and confines his movements to his own territory He becomes hostile 
to other males of the same species, with which he was on amicable 
terms only a short time previously. This bird drives others of his kind 
out of his territory, but pays little or no attention to birds of other 



Home Range and Territory 351 

species that wander by unless they actually intrude on his intended 
nesting site. Characteristically, the male bird chooses a singing 
perch, makes himself conspicuous, and, when a potential mate comes 
to the area, goes through the courtship display. In some species, as 
for example the house wren, the male may even start building a nest 
before the female arrives. (No doubt he has to do it all over again 
when she appears on the scene!) After the nest is completed and 
during the period when eggs or young are in the nest, the territory 
is defended with particular fierceness. 

By carefully observing the movements of a pair of birds for a 
period of time, the ecologist can determine the position of the unseen 
boundary lines of their territory. If the birds build a second or a 
third nest during the same season, these later nests are placed some- 
where within the original territory. The division of a region into 
territories by pairs of breeding birds is illustrated in the diagram of 
Figure 9.16. It will be noted that the territories of birds of the same 
species do not overlap indeed, a neutral zone usually exists between 
them but the territory of one species may overlap that of another 
species if no reaction of antagonism occurs between them. 

In a 16-hectare tract of spruce-fir forest in northern Maine 148 male 
birds of various species were found to have established breeding ter- 
ritories by a census conducted during a week in June. During the 
ensuing 3 weeks a total of 302 territorial males were removed from 
the tract by shooting. Study of the area indicated that new males 
from a "floating" unmated population moved in and established ter- 
ritories as rapidly as the former inhabitants were removed. Thus 
the number of sites suitable for the establishment of territories was 
shown to be the chief factor limiting the number of birds that could 
breed in the area (Stewart and Aldrich, 1951). 

The dimensions of the territory or home range may be measured 
in kilometers in the case of large mammals or birds of prey or in meters 
or smaller units in the case of smaller animals. Bears may roam over 
distances of 30 or 40 km, whereas porcupines confine their home 
activities to trips of a kilometer or so and the home range of beavers 
is ordinarily less than a kilometer in diameter. The home ranges of 
the whitetail deer vary in size from about 80 to 120 hectares ( 200 to 
300 acres), of the cottontail rabbit from 1 to 3 hectares, and of some 
kinds of mice from less than % to more than 1 hectare. The daily 
roamings of prairie deermice in southern Michigan were found to be 
confined within 2 or 2% hectares, but on different occasions during a 
breeding season both male and female deermice may travel over an 
area as great as 4 hectares (Howard, 1949). The home ranges of box 



352 



Relations within the Species 




Homing 353 

turtles in Paxtuxent Research Refuge, Maryland, investigated by 
Stickel (1950), were found to average 100 m in diameter and to over- 
lap considerably. Chameleon lizards were reported to establish ter- 
ritories of 20 to 25 sq m in area. The arrangement of territories of 
the wood ant is illustrated in Fig. 9.17. In this instance the ants* 
nests were found to be only a meter or so apart. The ants foraged 
for distances of 70 m or so, but in no instance did the members of 
one colony encroach upon the territory of another colony. In col- 
onies of breeding birds the private domain around each nest site may 
be very small indeed, but this may be jealously guarded (Fig. 9.18). 

Wood mice apparently do not establish territories that are actively 
defended, but home ranges are delineated. Some avoidance response 
keeps the animals confined to their home ranges. The reality of this 
action of population pressure has been demonstrated by trapping ex- 
periments. When an area is cleared of one species thus creating a 
"social vacuum" animals from neighboring areas will move into the 
vacant area within a few days. Obviously then, the animals on the 
periphery of the area in question did not fail to enter the area because 
it lacked food or other necessity, or because they lacked the inclina- 
tion to move the distances involved. The neighboring mice were 
kept out of the area by "social" pressure of some sort resulting from 
the presence of other members of the same species. Although animals 
avoid entering the territory of others, peripheral contacts with neigh- 
bors are presumably a normal condition. The rapid entry of the mice 
into a cleared area may be'clue in part to a movement of the animals 
"as if in an attempt to encounter again the stimuli produced by 
neighbors" (Calhoun and Webb, 1953). 

The size of the territory or home range is usually larger than that 
which would be needed for the food supply of the individual estab- 
lishing it. In the case of territories staked out at the beginning of 
the breeding season, it is obvious that the food supply must be ade- 
quate for the young that are to be raised. Apparently during the 
course of evolution reactions for establishing the territory have come 
into existence that cause the breeding pairs to space themselves out 
sufficiently to anticipate the food needed by the new family. The 
size of the home range or territory has probably come to be based 
primarily on the food supply, but in some instances it may be de- 
termined by available breeding sites or other needs of the species. 

Homing. Frequently the attraction of an animal to the location of 
its home is very strong; this is particularly true if the territory is 
established in relation to a breeding site, and the reaction is further 
intensified if young are present. The return of an animal to its ter- 



354 



Relations within the Species 




10 20 30m 



FIG. 9.17. Wood-ant trackways and nests at Picket Hill, Oxfordshire, England. 
= nests, o = former nests, B = birch trees, G = gorse bushes, x = nests of wil- 
low wrens. ( Elton, 1932, Copyright, Cambridge Univ. Press. ) 




Photo R. H. Heck, Am. Museum of Nat. Hist. 

FIG. 9.18. Territorial dispute in penguin colony, Bleaker Island, Falklands. 



Return Migration 355 

ritory or breeding site is referred to as homing. The reaction is par- 
ticularly familiar among birds, but other types of vertebrates, many 
species of insects, certain Crustacea, and some other invertebrates are 
known to home. Even the pulmonate snail Onchidium, after leaving 
its home crevice in the rocks to feed in the surrounding area, returns 
to its own particular crevice and cannot be induced to enter any other 
(Arey and Crozier, 1921). Bees fly unerringly back to the hive from 
distances greater than 6 kilometers. 

Homing is especially well developed among birds. The distances 
covered are often spectacular, and the precision with which the home 
territory is located is little short of miraculous. Although small perch- 
ing birds may travel only 100 m or so to procure food, hawks and 
eagles forage for many kilometers, and sea birds often range widely 
over the ocean before returning to the home site. When investigators 
have carried parent birds great distances away from their nests and 
released them in unfamiliar surroundings, the prompt return of the 
birds to their breeding sites has demonstrated the extraordinary 
strength of the homing reaction as well as the existence of some highly 
successful method of orientation. The fact that the performance of 
domesticated homing pigeons improves with training and familiarity 
with the terrain indicates that these birds use landmarks to some 
extent, but other methods of orientation must also be employed 
(Matthews, 1950). 

In nature birds will home across unfamiliar country and over water 
where no landmarks exist. Perhaps the most spectacular instance of 
homing is that of a Manx shearwater that was taken from its nesting 
burrow on the Island of Skokholm, Wales, banded for identification, 
and transported across the Atlantic by airplane. This bird was re- 
leased from the Logan International Airport, Boston, Mass., on June 
3, 1952, and arrived back at its burrow in Wales 13 days later, having 
traversed 3000 miles of ocean. We do not know whether birds and 
other animals use the same methods of orientation for homing as they 
do for seasonal migrations. The whole fascinating subject awaits 
further study. A general discussion of orientation was presented in 
Chapter 6. 

Return Migration. Many birds, mammals, fishes, and other animals 
move seasonally from one habitat to another, sometimes traversing 
great distances. Journeys of this sort in which at least some of the 
population completes a round trip are known as return migrations. 
The most spectacular movements of this sort are found among the 
vertebrates, but return migrations also occur in the invertebrates, as 
exemplified by certain crabs and by butterflies (Williams et al, 1942). 



356 Relations within the Species 

Among many species the instinct to return to a certain area is exceed- 
ingly strong, as will be realized, for example, by anyone observing fish 
migrating up a river. According to an early colonial report in New 
England: "Alewifes continue to move up the stream, yea though ye 
beat at them with clubs." Perhaps some of these migrations may be 
thought of as movements out from and back to a home territory and 
hence an extension of the homing reaction. Return migrations as 
adaptations to (1) climatic conditions, (2) feeding needs, or (3) 
breeding activity, have been discussed in previous chapters. 

In some of these migrations members of the population come back 
to the same general area or even to the same locality. Birds return 
to the same region after journeys of several hundred or several thou- 
sand miles and after absences of 9 months or more. Frequently a 
pair of banded birds uses the identical nest site for two or more sum- 
mers in succession although the birds have journeyed to the tropics 
during the intervening winters. The greater shearwater roams widely 
over the North and South Atlantic Oceans during most of the year, 
but its only known breeding place is on the Island of Tristan da Cunha 
about 2000 miles south of the equator (Murphy, 1936). In some 
similarly mysterious way slender-billed shearwaters ("mutton birds") 
from all over the Pacific Ocean locate two tiny islands between Aus- 
tralia and Tasmania and all descend on the islands on the same day 
in a huge flock to begin the breeding season (Griscom, 1945). 
Alaskan fur seals breed only on the Pribilof Islands in the Bering 
Straits, and they manage to locate in some unknown way these tiny 
bits of rock in the vast ocean after swimming for a 1000 miles or more 
from regions off the coast of California where they spend the winter. 

In some of these breeding migrations the reaction to the home ter- 
ritory may persist over the winter and stimulate the animal to return 
to the breeding area at the next season. It is difficult to believe, how- 
ever, that after several years in inland streams adult eels retain any 
homing reaction to the area in the Sargasso Sea where they were 
hatched yet they do return there. The same might be said of the 
Pacific salmon that ascend the rivers to spawn after spending 4 to 7 
years in the ocean, but something does stimulate the salmon to under- 
take the migration and something orients the fish to the very tributary 
in which they developed as larvae (Hasler, 1954). Perhaps migra- 
tions taking place after long intervals and also those correlated with 
climatic changes and feeding activities are unrelated to homing or only 
secondarily related to it. In these instances, external or internal fac- 
tors may stimulate the animal to migrate at a certain time, and condi- 
tions along the way may orient the animal back to the region of its 



Emigration 357 

origin with the retention of no specific horning reaction or of one that 
is operative only after the animal has arrived within the general area 
of its original home territory. 

Emigration 

When animals leave their home range never to return, the move- 
ment is spoken of as emigration. This type of migration may take 
place either as a drift of individuals or of small groups, or as a mass 
movement. The young of many species of birds and also to some 
extent the adults wander in all directions after the breeding season 
and before the autumnal migration to the winter range begins. An 
example of this centrifugal movement is obtained from the returns of 
banded ducks (Fig. 9.19). Drift emigration of this sort is very wide- 
spread in its occurrence, but it often escapes notice because the 
animals or birds move inconspicuously in small numbers. Some kinds 
of animals wander about for long periods of their lives, seeking pri- 
marily perhaps food or water, without definite return to any given 
place; such irregular roaming is spoken of as nomadism. This and 
other types of animal travels are discussed fully by Heape (1932). 

In contrast to the foregoing, mass emigration is often spectacular 
although it occurs in relatively few species. At more or less regu- 
larly recurring intervals such animals as the mouselike lemmings of 
Scandinavia and Canada, the springbucks of Africa, and the locusts 
of Egypt and India undertake mass movements involving millions 
or billions of individuals. The causes of these mass emigrations are 
not clearly understood: in some instances they may be due to climatic 
changes; in others they may be due to lack of food which may or may 
not be the result of overproduction of the species itself. The onset of 
acute hunger, nervous disorder, or other conditions occurring at the 
time of mass emigration often profoundly alter the behavior of the 
animals. For example, according to Heape (1932), some such in- 
ternal change "transforms the shy and timid springbuck into a fearless 
creature with no regard for danger of any kind, and converts the no 
less shy and timid lemming into a truculent swashbuckler." 

An instance of a mass emigration of lemmings was witnessed in 
1937 by the manager at the Perry River post of the Hudson's Bay 
Company, Northwest Territory, Canada, and an excerpt from his ac- 
count follows ( Gavin, 1945 ) : 

While camped at the old post site on April 27 and for about four 
days subsequently, the onset of the migration was noticed. The whole 
tundra was a mass of moving lemmings and each time we went into the tent 
there would be a dozen or more inside. The migration went on night and 



358 



Relations within the Species 




Lincoln, 1933 

FIG. 9.19. Radial emigration of ducks after the breeding season, plotted from 

reports of banded birds. 



Emigration 359 

day; my 13 dogs outside the tent could be heard killing them at frequent 
intervals during the night. 

When we started for the new post, the sea ice was covered with a moving 
mass of lemmings, all headed in an easterly direction. They stopped at 
nothing. Untold thousands plunged over the ice into the water of a lead, 
about a foot below, and swam the 10 or 15-foot channel, but were unable to 
climb up the sheet ice on the other side. They perished in large numbers 
in these leads, but here and there, they found passages up tne ice and 
blindly continued their journey. Around the ends of the leads they pressed 
on without interruption going through the pools of water, lying on the surface 
of the ice without deviation and without the slightest hesitation. This scene 
extended as far as we could see in any direction. The natives later in- 
formed me that at Kol-gyuak-a river 45 miles east of Perry the same thing 
was going on at the same time. I do not know how far west it extended, 
but it was the same for at least 15 miles west of the post. This mass 
migration lasted for about 10 days and reached its peak about May 3 or 4. 

In travelling from the old post to the new, my dogs grabbed up and ate 
so many lemmings while they were running that their stomachs distended 
to a noticeable degree. They were so surfeited that they were useless for 
further work until they had gotten over their abnormal feeding. I had to 
rest them for 24 hours to allow them to get over their gluttony. An esti- 
mate of the average density of lemmings during this migration would be 
one to the square yard. 

Studies of the migratory locust have shown that under ordinary 
conditions this insect exists exclusively in a form known as the soli- 
tary phase. Particular conditions of temperature or dryness cause a 
great increase in the numbers of the species, and this is followed by 
the development in the population of the swarming phase of the 
locust, differing in color and in structure from the solitary phase. 
With the appearance of the swarming phase, emigration of great num- 
bers of locusts begins (Fig. 9.20). Huge swarms of locusts, such 
as were recorded in Biblical times, spread into surrounding areas 
and do untold damage to crops and other vegetation. Since the 
newly invaded regions are less favorable for the maintenance of the 
species, fewer of the young survive, and numbers are also reduced 
by enemies that have been attracted by the plentiful food. As the 
density of the locust population diminishes, the swarming phase dis- 
appears and the solitary phase reappears. Gradually the locust dies 
out of the invaded areas, and the species shrinks back to its original 
range (Uvarov, 1931). 

In this chapter we have considered the wide variety of relations 
among members of the same species. Other individuals of the same 
kind become abundant in an organism's environment as a result of 
reproduction, of passive transport, or of active locomotion. We have 
seen that, as numbers of a species increase, eventually harmful effects 



360 



Relations within the Species 








Photos I). L. (i/ww. refixxIiHtd h\ firnnission of the Anti-Locust Research Centre 
FIG. 9.20. (Upper) A small swarm of migrating desert locusts in Kenya (density 
estimated at 14 insects per cu m). (Lower) Desert locust hoppers (fifth instar) 

marching. 



Emigration 361 

of competition, interference, or aggression always come into being. 
However, a moderate amount of crowding may be beneficial to 
members of a group, and this result may have favored the evolution- 
ary development of integrated social organization in animals. The 
fact that an increase in a population progressively curtails its own 
growth makes possible a mathematical formulation of population de- 
velopment and provides a basis for deducing the optimal yield that 
can be obtained by exploiting a population. Numbers of a species 
always fluctuate to a greater or a lesser extent. Every species has 
definite space requirements, determined either mechanically or by 
avoiding reactions related to home ranges, and overpopulation is re- 
lieved by the destruction of the extra individuals or by emigration. 
The presence of others of the same species thus forms a critical aspect 
of the environment, and each organism is involved in many reciprocal 
relations with individuals of its own kind. 



/o 

Relations 
between Species 



In natural situations the presence of other organisms of different 
species is an unavoidable and also a necessary part of the environment. 
The existence of other species may be crucially important in the pro- 
vision of food, shelter, or some other necessity. Contrarywise, vari- 
ous kinds of animals and plants are undesirable neighbors; but the 
presence of these species must nevertheless be dealt with as an in- 
fluence received from the surroundings. Certain interactions among 
species in an area are prominent and clearly discerned, but others 
are of a subtle nature not easily studied. Some of the relationships 
form an integral part of the operation of the ecological complex as a 
whole, whereas others may be of only minor consequence. In this 
chapter we shall continue our analytical approach to ecological prob- 
lems by surveying the various types of relationships between species 
and considering the operation of each separately. In subsequent 
chapters the combined operation of these interactions will be con- 
sidered in relation to the composition and adjustment of the commun- 
ity and to the functioning of the ecosystem as a whole. 

In attempting to delineate the various types of interrelations be- 
tween species one realizes that a great complexity exists. Animals 
have relations with other animals, plants with other plants, many 
animals are dependent upon plants in their environment, and some 
plants are dependent upon animals. All gradations exist from rela- 
tionships that are vital and lifelong to those that are casual and tempo- 
rary. Interdependency may exist between species of widely different 
kinds and sizes as between mighty redwood trees and microscopic 
bacteria, or in the animal kingdom between elephants and fleas. In 
some instances one species has an exclusive relation with another 
sometimes with one short life stage of the other species; but in other 
instances species are quite flexible in their dependencies upon their 
neighbors. 

362 



Relations between Species 363 

Interrelations between species may be beneficial to both parties, 
harmful to both parties, or beneficial or harmful to one and neutral 
in respect to the other. Every gradation may be found between these 
conditions. The beneficial effect of the presence of another species 
is sometimes a vital necessity; but in other instances in which only a 
trivial advantage is provided, decision is often difficult as to whether 
the relationship is actually beneficial or merely neutral. Positively 
harmful relations grade off in similar fashion to those that produce 
only a minor inconvenience or are essentially neutral. The nature 
of the relationship may change during the life cycle of one or both 
of the species concerned. Furthermore, as with intraspecific relations, 
the classification of an interaction between two species as beneficial 
or harmful depends upon whether consideration is given to the imme- 
diate effect on the individual or to the long-range effect on the species 
as a whole. In our present discussion we shall consider first the na- 
ture of interspecific relations as they act within the life span of the 
individual, and subsequently their more remote consequences to the 
success of the population and to the evolution of the species. 
j With the foregoing qualifications in mind, we may divide the in- 
terrelations between species into two main categories: (1) symbiosis, 
in which one or both species are benefited and neither species is 
harmed; and (2) antagonism, in which at least one of the species is 
harmed. Some authors extend the meaning of symbiosis to embrace 
all types of interrelations including harmful ones; other authors, taking 
the other extreme, limit the term to relations between a plant and an 
animal, or to relations that are mutually beneficial. The present defi- 
nition of symbiosis, which literally means "living together," seems 
more logical and more in keeping with established usage. Symbiotic 
associations are divided into those of mutualism (both species bene- 
fited) and commensalism (only one species benefited). Relation- 
ships of antagonism between species embrace antibiosis, exploitation 
(including parasitism and predation), and competition.^ 

Various ecological aspects of the foreging subdivisions will be dis- 
cussed in the following sections. E. F. Haskell's more elaborate clas- 
sification of "coactions" between species is discussed and illustrated 
by Burkholder (1952). A simplified arrangement of these inter- 
specific relations may be represented as tabulated on page 364. 
^j In this scheme ( -[- ) indicates an increase in a beneficial life process 
as the result of the association, ( ) indicates decrease or harm, and 
(0) indicates no significant effect. Since these relationships grade 
into one another and sometimes change, and since it is often difficult 
to determine whether the effect on one of the species is essentially 



364 Relations between Species 

neutral, attempts to classify many actual associations in these sub- 
divisions may not be profitable. The scheme is chiefly useful in clari- 
fying the types of relationship to be kept in mind while interspecific 
reactions are being examined. 

Species A Species B Relation 

+ + Mutualism Isymbiosis 

+ Commensahsm J 

Neutrality, toleration . 

Antibiosis | 

+ Exploitation (inc. parasitism and pro- > Antagonism 

(iation) J 

Competition 



SYMBIOSIS 

When members of two species are living together in a symbiotic 
relationship, the benefit received by one or both of them most fre- 
quently involves the provision of food, but it may also involve shelter, 
substratum, or transport. The association may be continuous or 
transitory, obligate or facultative. The two symbionts may be in close 
contact, with their tissues actually intermingled, or one partner may 
live within a cavity of the other or attached to its surface. In some 
instances contact between the individuals is transitory, and in some 
the two species may influence each other without actual contact. 
Associations in which both species derive benefit are termed mutual- 
ism; those in which only one species is benefited and neither is harmed 
are termed commensalism. 



Mutualism 

Mutua\\stn \vvt\v Continuous Contact T\\e most intimate type oi 
n\utY\a\\sm is seen in those associations in w\\ic\\ contact between the 
symbionts is close and is often permanent as well as obligatory. A 
classic example of this sort of relationship is furnished by the lichens, 
which are composed of a matrix formed by a fungus within which 
cells of an alga are embedded. The fungus holds moisture and makes 
minerals available for both partners, in return for which the chloro- 
phyll-bearing alga manufactures carbohydrates for itself and also for 
the colorless fungus. The fungus of a lichen can never grow in nature 
without the associated algae, and the algae, although generally similar 
to independent species, are probably dependent upon the fungus 
under natural conditions. The dual personality of the lichen may be 



Mutualism with Continuous Contact 



365 



demonstrated in the laboratory by suitable techniques that permit the 
algal and fungal components to be cultured separately, as is sug- 
gested in Fig. 10,1. Many lichens grow abundantly on bare rock 
surfaces where the lack of moisture and of organic matter would make 
life impossible for most independent algae and fungi. 

Other equally fascinating instances of inutualistic symbiosis in- 
volving two plant species are to be found in nature. A well-known 
example, and one already mentioned as of great ecological importance 
relation is nutrients, is furnished by the bacteria of the genus 



in 



Mature lichen 



Ascospores 




Pure cultures 



Clasping hyphae^- 



Association 



Lichen is formed in nature but 

rarely in laboratory 

FIG. 10.1. Diagram illustrating the symbiotic partnership represented by the 
lichen Cladonia cristatella and its separation in the laboratory into algal and fungal 
components. When the components are recombined in the laboratory, the typical 
morphology of the lichen is not developed, evidently because of the lack of certain 
environmental conditions found in nature. ( Burkholder, 1952, Copyright Baitsell's 
Science in Progress, Yale University Press.) 



366 Relations between Species 

Rhizobium, which form nodules on the roots of leguminous plants and 
live symbiotically with their hosts (Fig. 10.2). The bacteria are 
somewhat specific as to host, and obtain carbohydrate and other sub- 
stances from its juices. In return the rhizobia fix gaseous nitrogen 
and pass it on to their plant host. If available fixed nitrogen is absent 
from the soil, the plant is completely dependent upon its symbiotic 
nitrogen-fixing bacteria. 




FIG. 10,2. Vertical section through the root of a soybean plant, showing the root 
nodules within which live symbiotic bacteria (Rhizobium japonicum). 

A somewhat similar mutualism is seen in the fungi that form mycor- 
rhizal structures either inside the roots of certain plants or on their 
outside surfaces (Fig. 10.3). Ectotrophic mycorrhizae are found on 
various kinds of trees such as pines, oaks, hickories, and beech; endo- 
trophic mycorrhizae occur in the red maple and are particularly com- 
mon in roots and other tissues of many orchids and heaths. Fungi of 
this type are nourished by organic material that they absorb from 



Mutualism with Continuous Contact 367 

their hosts. The ectotrophic mycorrhizae commonly take the place of 
root hairs and function in the absorption of water and nutrient salts 
from the soil. The degree of dependency of the host plant upon 
mycorrhizae is very variable, but pine seedlings, at least, appear to be 
quite unable to grow in soils normally deficient in one essential nutri- 
ent without the aid of these symbiotic structures. Since the bene- 
ficial mycorrhizae of the blueberry, rhododendron, and heather 




Photo by Somerville Hastings, from McDougall, 1949, Copyright, Lea and Febiger 

FIG. 10.3. Ectotrophic mycorrhizae seen as whitish sheaths over the branching 
rootlets of the hornbeam (Carpinus betulus) growing in leaf mold. 

flourish only in an acid medium, the growth of these plants is im- 
proved by a soil of low pH (McDougall, 1949). It is reported that 
settlers moving west in the United States were at first unsuccessful in 
establishing certain kinds of trees around their homesteads because of 
the absence of suitable fungi. The seeds that they brought with 
them sprouted, but often the young trees would not grow. When 
fungal spores accidentally reached the plantations in soil samples car- 
ried from the east, many of the necessary mycorrhizae were supplied 
and better tree growth became possible. Subsequently deliberate 
inoculation was practiced, especially for the growth of pine trees. 

Even more remarkable associations involving the intermingling 
of tissues are those in which one partner is an animal and one a plant. 
Unicellular plants live symbiotically in the outer tissues of certain 



368 Relations between Species 

sponges, coelenterates, mollusks, and worms. Some of these uni- 
cellular forms are green algae known as zoochlorellae; others are 
brown or yellow cells, believed to be flagellates, and are termed zoo- 
xanthellae. Partnerships called plant-animals by Keeble (1910) are 
formed by a turbellarian worm (Convoluta roscoffensis) and large 
numbers of zoochlorellae. The algal cells, often growing in the tissues 
of the worm in such profusion as to give it a greenish appearance, 
release oxygen during photosynthesis and produce nitrogen com- 
pounds that are nutritionally beneficial to the host. In exchange the 
algae obtain a suitable matrix for their growth and receive a supply of 
nutrient material resulting from the animal's metabolic processes. 
This balanced symbiotic relationship is thus able to persist indefi- 
nitely, and the nourishment furnished by the algae enables the worm 
to live and grow for long periods without taking in solid food. 

Zooxanthellae are found abundantly in the body wall of coral 
polyps. These unicellular organisms serve the useful function of 
removing nitrogenous wastes and carbon dioxide from the coral and 
providing it with oxygen produced as a by-product of photosynthesis. 
The zooxanthellae in turn benefit by the absorption of the metabolites 
containing nitrogen and phosphorus, which are scarce in tropical 
waters, as well as by the absorption of carbon dioxide resulting from 
the catabolic processes of the coral animal. The fact that coral polyps 
placed in the sea in sealed glass containers survived for 2 weeks is 
evidence that the symbionts in this instance had approached a state of 
balance, in regard to their respiratory exchanges at least, and perhaps 
also to some extent in regard to other needs. 

The giant clam Tridacna (Fig. 7.11) grows on shallow coral reefs 
with the opening between its shells directed upwards and the broad 
edges of its mantle, containing vast numbers of zooxanthellae, spread 
out horizontally where they receive intense radiation from the sun. 
The mantle also contains great numbers of small lens-like organs 
which probably serve to focus light into the tissue, making photo- 
synthesis possible for the deeper-lying /ooxanthellae. Since phagocy- 
tic blood cells regularly engulf and digest large numbers of the zoo- 
xanthellae, the giant clam may be thought of as "farming" these sym- 
biotic algae and deriving a considerable portion of its nutrition from 
them (Yonge, 1944). 

Some symbionts reside in cavities of their hosts rather than in their 
tissues. Ruminants and other animals living on a diet high in cellu- 
lose are unable to digest this material without the enzymatic action 
of cellulase produced by microorganisms in their intestines. Sym- 
biotic bacteria fulfill this function in cattle and other grazing animals. 



Mutualism with Continuous Contact 



369 



Certain cockroaches and termites can digest wood only with the aid 
of a special type of flagellate that is harbored within their guts. 
Symbiosis in these instances is mutually beneficial and obligatory for 
both parties. Some of the bacteria living in the intestines of animals 
also produce various B vitamins and other special materials. 

Another manifestation of mutualism in which the symbionts are in 
permanent contact, but one in which the contact is entirely external 
is the attachment of certain marine sponges and coelenterates to the 
shells of crabs. The attached animal benefits by being carried about 
to fresh feeding areas and by avoiding being stranded in the tidal 
zone or in stagnant water, as well as by obtaining fragments of food 
from the meal of its host. The crab, for its part, is camouflaged to 
some extent by the presence of the attached animals on its back and 
is often protected by them from attacks by its enemies. 




FIG. 10.4. Three sea anemones attached to the shell of a hermit crab, illustrating 

mutually beneficial symbiosis. Note the growth of the foot of the anemone over 

the surface of the shell. ( Modified from Borradaile, 1923. ) 

The classic example of this type of partnership under the sea is 
furnished by the sea anemone Adamsia palliata which grows on the 
shell of the hermit crab Eupagurus prideauxi (Fig. 10.4). Further 
interspecific relations result in this instance from the fact that the shell 
inhabited by the hermit crab is the abandoned house of a snail and 
from the fact that the same shell may also furnish the abode of the 
annelid worm Nereis. The hermit crab starts the enterprise by ob- 



370 Rclatiotis between Species 

taining a sea anemone from a rock and placing it on the back of its 
shell. When the growth of the crab causes it to move to a larger 
shell, the crab loosens the base of the anemone with its claws and 
transfers it to the new shell. In some species the base of the anemone 
grows over the entire shell and extends beyond it; sometimes the 
original shell is largely dissolved away so that the crab's house con- 
sists almost entirely of the anemone's base. The stinging nematocysts 
of the sea anemone are a powerful deterrent to predaceous fish and 
protect the members of the partnership from being eaten. When 
Nereis is also present as a junior partner, it is said to help keep the 
inside of the shell clean and, as payment, to snatch fragments of food 
from the pincers of the crab. 

Mutualism without Continuous Contact. A great many instances 
exist of mutualism in which the partners are not attached to each other 
or in which they are in contact intermittently or for only a short pe- 
riod. In most mutual benefit associations of this sort the fulfillment 
of a nutritional need plays a prominent role for at least one of the 
species; but the other species, and sometimes both, may derive an 
entirely different type of advantage. 

A commonly cited example of mutualism of two animal species in- 
volves birds that alight on the backs of large grazing animals and 
pick off the ticks or other external parasites. The cowbird in North 
America, the oxpecker, the little white heron in Africa, and certain 
other birds obtain a ready supply of food in this way. The host ani- 
mals are rid of their pests and are frequently warned of approaching 
danger by the activity of the birds as watchmen. An amazing kind 
of pest-control service is rendered by the crocodile bird in removing 
leeches from around the teeth of the crocodile, which allows the bird 
to enter its mouth for the search. Mixed groups of ostriches and 
zebras are said to derive mutual benefit in guarding against attack 
by the keener sense of sight of the ostrich and the greater powers of 
scent possessed by the zebras. 

In the subterranean world we find various extremely complex re- 
ciprocal relations between species. Various ants maintain a popula- 
tion of aphids in their nests. The ants obtain a nutritive exudation 
from the hind end of the alimentary tract by stroking the aphid's 
Abdomen with their antennae. This furnishes the basis for popular 
accounts that ants "keep cows" and "milk" them. The aphids feed 
on the roots of plants, or they are carried by the ants out of the nest 
and allowed to feed on leaf stalks. The aphid eggs are laid on the 
plants above ground, and, although of no immediate use to the ants, 
they are carried down into the nest where they are sheltered during 



Mutualism without Continuous Contact 371 

the winter until they hatch and the young aphids repopulate the dairy 
farm (Fig, 10.5). 

An equally extraordinary agricultural technique practiced by in- 
sects is the cultivation of fungi by certain beetles, ants, and termites. 
Various kinds of fungi are grown for food by these animals, and some 
species are known only in insect gardens of this sort. In the tropical 
forest one may see a band of green moving across the ground; this is 
formed by a line of leaf-cutting ants (Atta) carrying pieces of leaves 




FIG. 10.5. Diagram of an ant nest containing a "dairy farm" of aphids. ( Burk- 
holder, 1952, Copyright, Baitsell's Science in Progress, Yale Univ. Press.) 

over their backs like so many umbrellas. The leaves have been cut 
from a shrub and are being transported to the ant's underground nest 
where they will be chewed into a pulp and spread out to form a bed 
in which a particular kind of fungus (Rozites gongylophora) is 
planted. The ants cultivate the garden with great care; they weed 
out unwanted species of fungi and prevent the fruiting of their 
fungus, but encourage it to develop special mycelial outgrowths on 
which they feed ( Brues, 1946 ) . The fungi that live symbiotically in 
termite nests are nourished by the insects' excreta rather than by leaf 
pulp. The cultivation of fungi by ambrosia beetles (Scolytidae) is 
carried on within the tunnels drilled in wood by these insects ( Cham- 



372 Relations between Species 

berlin, 1939). Sometimes the development of the fungi is sufficiently 
luxuriant to clog up the tunnels completely, and the beetles are killed 
unless they can eat their way out faster than the fungi grow! 

The perpetuation of this mutualistic relation between insects and 
fungi is assured by elaborate structures and reactions by which the 
fungi are transferred to the habitats of new generations. The fungus- 
growing beetles, for example, possess certain external or internal 
structures by means of which they carry spores or fragments of the 
fungi from the old burrow, where the larvae were hatched, to the site 
of a newly founded colony. Among the leaf-cutting ants the virgin 
queen carries a pellet of fungus in a pocket below the mouth and 
deposits the inoculum in her new bridal chamber. These arrange- 
ments by the host for transmission are strong evidence that the guest 
species are beneficial, and hence are symbionts. With parasitism, the 
problem of transmission is always arranged for by the parasite in one 
way or another and is resisted by the host. A more detailed consid- 
eration of these relationships among insects will be found in the sum- 
mary by Steinhaus ( 1946 ) . 

The pollination of flowers by bees, moths, and butterflies, and oc- 
casionally by hummingbirds, is another manifestation of mutually 
beneficial symbiosis, but one in which the species concerned may be 
in contact for only a few seconds. The insect derives food from the 
nectar, or other product of the plant, and in return carries pollen from 
the anthers of one flower to the stigma of another, thus ensuring cross 
pollination. v The coordination of the elaborate behavioristic and 
anatomical adaptations that have been evolved in the insect and in 
the flower in relation to this cooperative activity is truly remarkable. 
The flowers open, producing odors and displaying colors that attract 
suitable insects, only at the time when they are sufficiently mature for 
fertilization. In various ingenious ways the flower is so shaped that 
the insect cannot get its food without dusting the stigma with pollen 
carried from another flower and then picking up fresh pollen to be 
carried to the next flower. The reactions and the structure of the 
pollinating insects are correspondingly adapted. Bees, for example, 
continue to visit the same species of flower as long as a supply of 
mature blossoms is available and thus avoid mixing pollen from dif- 
ferent species. 

The mutual dependence of insect and flower is frequently highly 
specific and sometimes involves the reproductive cycle of the animal 
as well as that of the plant. The peculiarly enclosed flowers of the 
commercial fig are pollinated only by wasps of the genus Blastophaga, 
and special floral structures called caprifigs provide the only place in 



Mutualism without Continuous Contact 



373 



which these wasps can lay their eggs. A similar dually obligate and 
specific symbiosis is found in the relation between the yucca plant 
and the yucca moth (Fig. 10.6). The female moth visits the yucca 
flower in the evening and collects a ball of pollen from the anthers. 
Then, holding the pollen ball in specially adapted mouth parts, she 
flies to another plant and pierces the ovary of the flower with her 




Fie. 10.6. The Yucca moth (Pronuba ijnccasella) approaching flower of the 

yucca plant (left). Flower cut open (right), showing the moth placing pollen on 

the stigma. ( Modified from Borradaile, 1923. ) 

ovipositor. After depositing eggs within one of the ovules, the moth 
creeps down the style and stuffs the ball of pollen into the stigma. 
When the moth eggs hatch, the larvae feed on the tissue of the ovule, 
and eventually mature to repeat the process for the next generation. 
It is difficult to imagine in such complex intcrdependencies how the 
behavior pattern and anatomical structures vitally necessary to both 
species have evolved. 

Other two-way benefits in the relations between species involve 
transport and dispersal of seed. The fruit eaten by birds, mammals, 
and other animals provides them with a source of nourishment, and 
the contained seeds are subsequently dropped in their excrement at 
varying distances from the original site. The activity of squirrels in 
carrying and burying acorns, hickory nuts, and the like may play a 



374 Relations between Species 

significant role in the establishment of trees in new areas. Many 
other illustrations of mutualism, both of an intimate and of a casual 
nature, will occur to the reader. 

Commensalism 

When members of different species are associated in such a way 
that only one of the organisms is benefited but neither is harmed, the 
relationship constitutes commensalism. ^ Such associations no doubt 
began by the mere toleration of "guests" near the usually larger host 
species, or on or in its body. If the guest derived some benefit with- 
out interfering with the host, the relationship would tend to persist. 
Casual association may have led to a partial or a complete dependency 
on the part of the guest. Obligate commensalism established in this 
way may have evolved further in some instances to give rise to mutual- 
ism, on the one hand, or parasitism, on the other. If the host species 
became adapted to take some advantage of the close proximity of its 
guest, a mutualistic symbiosis would result. However, if during the 
course of evolution the guest species imposed more and more upon 
its host, finally overstepping the bounds of hospitality and inflicting 
harm upon the host, the relationship would change to exploitation and 
perhaps to parasitism. 

The advantage derived by the commensal involves the provision 
of substratum, shelter, or transport, and very frequently of food. 
Commensalism means "eating off the same table" as guest messmates. 
"The messmate does not live at the expense of his host; all that he 
desires is a home or his friend's superfluities" (Pearse, 1939). As 
would be expected, the circumstances of commensalism are more 
variable than those of mutualism. Although in some instances the 
commensal is in continuous contact with its host attached to a sur- 
face or retained within a cavity more frequently the guest is free to 
come and go at irregular intervals. Sometimes the commensal species 
can associate with only a single host species, but often considerable 
species flexibility is observed. 

Commensalism with Continuous Contact. Commensals in more or 
less permanent contact with their hosts are represented by a great 
variety of epiphytes and epizoans. Many tropical orchids, brome- 
liads, and other "air plants" grow perched on horizontal branches or 
in forks of trees or hanging in streaming festoons (Fig. 10.7). Fa- 
miliar in the northern coniferous forest are the hanging "mosses" 
Usnea and Alectoriattliat gave rise to Longfellow's well-known lines: 



\j 



Commensalism with Continuous Contact 



375 



This is the forest primeval. The murmuring pines and the hemlocks, 
Bearded with moss, and in garments green, indistinct in the twilight, 
Stand like Druids of eld, with voices sad and prophetic, 
Stand like harpers hoar, with beards that rest on their bosoms. 

Each of these mosses consists of two species forming a lichen, and 
this grows as an epiphyte upon the conifers, thus creating a three-way 
partnership. Epiphytes sometimes show distinct preferences for one 
host species, as may be seen, for example, in southeastern United 




Photo by H. B. Moore 
FIG. 10.7. Bromeliads, including the pendant Spanish moss ( Tillandsia ) , growing 

as epiphytes on the branches of the live oak in Florida. 

States where the Spanish moss grows more abundantly on the live 
oaks than on the pines (Fig. 10.8). 

All these epiphytes use the trees only as a point of attachment amid 
suitable light and other conditions, and manufacture their own food 
by photosynthesis. Since they do not obtain nourishment from the 



376 Relations between Species 

tissues of the tree, they are not classed as parasites. For the most 
part the epiphytes do no harm to the host plant but occasionally they 
become so numerous as to break it down or to. stifle its growth ( Fig. 
10.8). Orange growers in Florida are forced to spend large sums 
every year for the removal of Spanish moss from their trees. 




FIG. 10.8. Spanish moss (Tillandsia) growing in harmful abundance on a live 
oak tree in South Carolina but practically absent from a neighboring pine. 

Some plants live as attached epiphytes on the surfaces of animals. 
One extraordinary example is the green alga that grows on the long, 
grooved hairs of the sloth. Since this alga often becomes sufficiently 
abundant to give the animal a greenish appearance, the sloth pre- 
sumably derives some advantage in concealment as it sleeps in the tree 
tops. Equally remarkable in its habit is the green alga Basicladia, a 
genus of the Cladophoraceae, which grows only on the backs of fresh- 
water turtles (Leake, 1939). The specific relation here appears to 
be due to a dependence on keratin, since the alga can be cultured in 



Commensalism without Continuous Contact 377 

the laboratory only if this substance is provided. To the extent that 
the host receives benefit from such epiphytic algae, the relationship 
approaches mutualism. 

Many kinds of microorganisms take up residence within tissues or 
cavities of larger plants and animals without causing any trouble for 
the owner but without paying any rent. Since no light is available 
for photosynthesis these commensals are represented by saprophytic 
fungi and bacteria and by Protozoa. Many such organisms are 
found in the lower intestines of animals where they consume un- 
digested food and secretions and complete their life cycles unnoticed 
by their hosts. The bacteria in the human colon, notably Escherichia 
coli, are a familiar example. As already mentioned, these commen- 
sals may represent a mode of life that is transitional between that of 
the parasites and the beneficial symbionts also found in such habitats. 

Permanently fixed commensals in the animal kingdom are repre- 
sented by sessile invertebrates that grow attached to plants or to other 
animals. Sometimes a highly spedific relation exists between the 
epizoan and its host, although the reason for this is often hard to dis- 
cern. The oyster-like bivalve Ostrea frons grows almost exclusively 
on the roots of the red mangrove in the shallow waters off the coast 
of Florida; special hooks develop on the lower shell of this animal by 
means of which it clings tenaciously to its host and is not displaced 
by surf. Certain barnacles are found only on the backs of whales 
where they benefit by a free ride; since they do not feed upon the 
whale's flesh, they are not parasites in the strict sense of the term. 
The marine environment furnishes many other instances of attached 
commensals (Wilson, 1951, Ch. 11). Anyone who has an opportunity 
to catch an elderly horseshoe crab (Limulus polyphemus) in the 
shallow water off the New England coast is likely to find several 
species of mollusks, barnacles, and tube worms attached to the shell 
and a number of more motile commensals living in the "book gills" 
or other anatomical nooks of this strange animal. 

Commensalism without Continuous Contact. The category of 
commensals that are in temporary contact with their hosts or that are 
associated without being in actual contact is also a large one. Inter- 
mittent contact between animal commensals and plant hosts is dis- 
played on land by squirrels, monkeys, tree frogs, and snakes, and by 
a great many birds, insects, and other animals that use trees or other 
plants for substrata, for shelter, or for breeding sites without harming 
the host plant significantly. yMany examples of partial or complete 
specificity in Commensalism of this type will occur to the reader. An 
intriguing three-cornered relationship is that of the elf owl that nests 



378 Relations between Species 

only in abandoned holes made by the Gila woodpecker in the stems 
of the large Sahuaro cactus found in Arizona and neighboring re- 
gions. 

Temporary or intermittent contact between two animal commensals 
is seen in such special associations as that of the remora fishes and the 
sharks, whales, or sea turtles to which they attach. The dorsal fin of 
this "shark sucker" has become modified in the course of evolution into 
a most effective suction disc by means of which the fish attaches itself 
to the under side of a shark or other large animal. The sucker can 
release its grip at will, swim about gathering fragments of food re- 
sulting from the shark's meal, and return to hook another ride on the 
body of the host. Several suckers may be found attached to the 
same shark, but they do not seem to hinder the powerful fish ap- 
preciably. 

Commensalism involving close association between two species but 
without the attachment of the guest to the host occurs in a wide 
variety of animal groups. The guest may live in a burrow or other 
retreat of the host species, and sometimes the relation is highly specific. 
Certain beetles, for example, are known exclusively from the nests 
of meadow mice. The burrowing owl which often nests in the bur- 
row of a prairie dog is an accepted member of the prairie dog "village" 
while the village is actively populated by these rodents. Many such 
uninvited guests take up residence in the burrows or tubes of aquatic 
animals. The decapod crustacean Polyonx lives in the "back entry" of 
the U-shaped tube of the marine annelid Chaetopterus, where it is 
well hidden from enemies and where it can obtain particles of food as 
well as a supply of oxygen from the water forced through the tube by 
the pumping action of the worm's parapodia (Fig. 10.9). 

Other marine commensals live within a water cavity of their host. 
In certain instances in which the relationship is highly specific the 
commensal has been shown to be guided to the proper host by chem- 
ical emanations or to be kept within the host by definite tactic re- 
actions ( Davenport, 1950 ) . The oyster crab is a familiar example of 
this type of commensalism and one among several of a similar nature 
in which the commensal eventually becomes a prisoner. The oyster 
crab is originally carried as a planktonic larva into the mantle cavity 
of the oyster by incoming water currents produced by the feeding and 
respiratory activity of the host. The tiny larva metamorphoses, and 
eventually grows into an adult about a centimeter in length and too 
large to escape through the narrow opening between the valves of 
the oyster. The crab leads a completely sheltered life, stealing par- 
ticles of food from the oyster, to be sure, but apparently doing its 



Commcnsalism without Continuous Contact 



379 



host no significant amount of harm. The commensal is still within 
the cavity of the shell when the 1 oyster is opened in the kitchen, and 
the crab sometimes startles an uninformed diner by appearing in his 
oyster stew. (Oyster crabs are perfectly edible and. are regarded 
as a delicacy by some, including many who are not ecologists.) 

Perhaps the most extraordinary instance of a commensal living 
within its host is that of the small tropical fish Fierasfer which finds 



^.k4ffcL'|iti* - 




FIG. 10.9. Diagramiuatic vertical section of a Chaetopterus tube in the mud 
bottom of the littoral zone, showing the position of the worm and two commensal 
crabs (Polyonyx). (By permission from Animal Ecology by A. S. Pearse, Copy- 
right 1939, McGraw-Hill Book Co.) 



380 Relations between Species 

shelter within the cloacal cavity of a sea cucumber ( Holothuroidea ) . 
The fish occasionally emerges to feed in the neighborhood. When it 
wishes to reenter its strange retreat, it pokes its nose against the open- 
ing of the cucumber's cloaca, then quickly reverses its position, and 
allows itself to be drawn tail first inside its host. 

Some species appear to derive protection by acquiring a big brother, 
that is, by associating themselves with other species recognized in the 
community as being voracious or poisonous. Pilot fish (Naucrates 
ductor) follow along beneath sharks as closely as shadows, but can- 
not attach to the surface of their hosts since they have no suction 
discs as do the remoras. For some reason they are never eaten by 
the sharks. In similar fashion particular kinds of fish find shelter 
under the umbrella of poisonous jellyfish; one species somehow man- 
ages to live unharmed among the tentacles of the Portuguese man- 
of-war, whose stinging cells instantly paralyze other kinds of fish, 
which are used for food. Another commensal fish has acquired a 
type of swimming motion that allows it to move among the tentacles 
of large sea anemones without causing the discharge of the poisonous 
nematocysts. 

Other instances of the protection gained by the weak through as- 
sociation with the strong are furnished by scavengers and mimics, al- 
though their modes of life should perhaps not be termed commensal- 
ism because of the absence of toleration or even awareness. In the 
tropics the hyena lives as a scavenger on leavings from the lion's meal, 
which consists of animals that the hyena could not possibly have 
killed for itself. Similarly, in the far north during the winter the 
arctic fox lives largely upon the remains of seals killed by polar bears. 
Another type of protection is attained by weaker species through 
mimicking in appearance and behavior certain species recognized as 
voracious or poisonous by other members of the community, as de- 
scribed in Chapter 6. The mimics, as well as the truly dangerous 
models, are presumably avoided by the predators. 

The foregoing account of symbiosis is sufficient to indicate the 
widely separated types of animals and plants that may be associated 
in relations of advantage or of toleration. We have also reviewed 
the great range that exists in the degree of dependence and of specific- 
ity in these relationships. However, many fascinating questions 
await future study as to the exact nature of the behavior patterns that 
bring the symbionts together and of the physiological interdepend* 
encies between them. The examples described show how these asso- 
ciations may have arisen from casual contacts between species, which 
then led to a recognizable commensalism. Subsequently the relation 



Antagonism 381 

may have evolved in some instances into mutualism and in others into 
parasitism. The possibility of evolutionary transformation in either 
direction may be imagined from such flexible associations as those of 
the various mycorrhizae. 

The interspecific relationships described in this section are flexible, 
and their nature may change even within the lifetime of the individ- 
uals concerned. At one moment the tissue of the giant clam is pro- 
viding a favorable environment for the growth of zooxanthellae; at 
another moment it is digesting these unicellular organisms. Fungi 
associated with the roots of the tomato plant are ordinarily harmless, 
or even beneficial; but under certain conditions these fungi become 
definitely parasitic on the same plant. Another turn-about relation- 
ship, which will be more fully described in Chapter 12, occurs in 
stands of "old field" pine. The grass in abandoned fields in central 
New England forms a favorable bed for the development of seeds 
that blow in from white pine trees. When the pines have grown into 
a dense grove, however, the resulting deep shade and thick carpet of 
fallen needles will kill the grass beneath. These examples will suf- 
fice to indicate how easily the line between symbiosis and antagonism 
may be crossed. 

ANTAGONISM 

Relations between members of different species in which one or 
both are harmed during the life span of the individuals concerned 
are included under the general heading of antagonism. These antag- 
onistic relations are not necessarily harmful for the population or for 
the species as a whole. From the broader viewpoint interspecific 
antagonism may have beneficial effects through controlling abun- 
dance or influencing the course of evolution V Exploitation and com- 
petition are necessary for the very existence of the community, and, 
when kept in suitable balance, make possible its perpetuation.^ Our 
scrutiny of the relations between individuals of different species there- 
fore will lead to a consideration of the relations between interacting 
populations. The latter are involved in functional adjustments of the 
community as a whole which will be discussed in subsequent chap- 
ters. As with other ecological factors, when the interaction between 
species is extended from individuals to populations and the entire 
community, additional relations with new significance emerge. 

Antagonistic relations of one species toward another involve harm 
that may be inflicted in one or more of several ways: (1) Species 
A may produce a poisonous substance or a change in environmental 
condition that is inimical to species B without species A deriving any 



382 Relations between Species 

benefit (0, ). Antibiosis; the production of harmful secretions, is 
the outstanding example of this type of relationship. (2) Species A 
may inflict harm by direct use of species B for its own benefit in a 
relationship of exploitation^ -f-, ). Species A may exploit B by 
gaining support or shelter for itself, or it may obtain nourishment di- 
rectly from B as a parasite or as a predator.* (3) Both species may be 
harmed in a reciprocally unfavorable relationship ( , ). Thijj 
situation is commonly found in the indirect rivalry, or competition, of 
two species for some feature of the environment that they both need 
and that exists in short supply. The space, food, light, oxygen, or 
other necessity taken by one species reduces the amount available for 
the other. In some situations considerable harm is suffered by both 
competing species; in other instances the injury may be serious for one 
species and relatively minor for the other. A reciprocally harmful 
effect will also occur if species A produces an antibiotic harmful to 




FIG. 10,10. Roots of strangling fig enveloping the trunk of its host in a Florida 

hammock. 



Antibiosis 383 

species B and at the same time an antibiotic secreted by B is harmful 
to A. v 

In the foregoing paragraph the term competition was used in a re- 
stricted sense referring to the indirect rivalry of two species striving 
for some necessity from a limited supply. The aggression between 
species that prey on one another or exhibit some other form of direct 
antagonism may also be considered competition in the broad sense. 
The ecological effects of competition as both direct and indirect 
antagonism will be considered both from the point of view of the in- 
dividual and of the population and the species. 

All gradations exist between relations that are harmful to one 
species and neutral to the other, those that are harmful to one and 
beneficial to the other, and those that are harmful to both parties. 
Associations between species that are at first neutral or even bene- 
ficial may change during the life of the individuals to become harmful 
to one species, as already mentioned* and this relationship may change 
further to bring disadvantage to both parties. For example, the 
strangling fig starts life as a harmless epiphyte on a palm or other 
tree, but after a time its dangling roots reach the ground. Deriving 
nutrients from the soil and energy from sunlight, the fig rapidly en- 
velops the trunk of its host with a network of anastomosing roots 
(Fig. 10.10). The luxuriant growth of the upper part of the fig grad- 
ually shades out the palm, and, after the death of the host tree, the fig 
eventually falls to the ground from lack of support. In similar fashion 
a parasite or a predator may bring about its own destruction under 
certain circumstances by killing its host or consuming all of its prey. 

Antibiosis 

Many substances produced by organisms, or conditions resulting 
from their metabolism, are generally harmful to other sr Thus the 
generation of carbon dioxide or of organic acids may harm more sen- 
sitive species to such an extent that they are unable to continue to 
live in the area^and excessive shading by one kind of vegetation will 
kill off species intolerant of low illumination.*' The term antibiosis 
applies more particularly to the production of materials that are 
specifically antagonistic to other speciesXSuch antibiotic substances 
are known to be generated by many kinds of fungi and bacteria.^ The 
action of penicillin, streptomycin, aureomycin, and other antibiotics 
produced by fungi in destroying various pathogenic bacteria is fa- 
miliar to the reader through the practical use of these substances 
either natural or syntheticin medicine. * 



884 Relations between Species 

Bacteria, molds, and actinomycetes that produce antimicrobial sub- 
stances have now been shown to be widespread in nature. Burk- 
holder ( 1952 ) reports that about half of the species of actinomycetes 
and half of the lichens as well as large numbers of higher plants pro- 
duce substances that inhibit certain molds and bacteria. Under 
some natural conditions antibiotics are believed to protect the species 
producing them from bacterial marauders, but this ecological aspect 
of the subject is largely unexplored. 

Antagonistic substances formed by algae have been shown to af- 
fect other algae adversely in the laboratory and undoubtedly also do 
so in nature under some circumstances. Some substance accumulat- 
ing in cultures of the green alga Chlorella vulgaris was found to in- 
hibit the growth of the diatom Nitzschia frustrulum to a greater ex- 
tent than its own growth; similarly, an antibiotic produced by dense 
cultures of Nitzschia retarded the division rate of Chlorella grown in 
the same culture or in conditioned water filtered off from the diatom 
culture. Interspecies antagonism of this sort probably exerts control 
on the abundance of different kinds of phytoplankton in ponds or 
other aquatic areas, and in some instances it may influence the sea- 
sonal sequence of species commonly observed in nature ( Rice, 1949 ) . 

Similar products of plant growth have been found to be harmful 
to certain animals. Substances produced by senescent cultures of 
Chlorella and of the diatoms Navicula and Scenedesmus inhibit the 
filter feeding of Daphnia in laboratory tests. A reaction of this sort 
may account in part for the curtailment of the growth of zooplankton 
sometimes observed in natural waters containing senescent popula- 
tions of phytoplankton (Ryther, 1954&). Pond "blooms" of blue-green 
algae, especially of the genus Microcystis, are known to produce toxic 
substances, such as hydroxylamine, which cause the death of fish and 
even of cattle that drink the water (Prescott, 1948). In the marine 
environment toxins produced by huge populations of certain micro- 
organisms, popularly known as "red tide," cause the catastrophic de- 
struction of fish and other animals. An outbreak of the dinoflagellate 
Gymnodinium brevis off the west coast of Florida in 1946-1947 re- 
sulted in the wholesale death of fish throughout an area of several 
thousand square miles (Gunter et al., 1948). 

Certain land plants are similarly harmful to animals, but they usu- 
ally exert their poisonous effects only when eaten. For example, a 
semidesert bush (Halogeton glomeratus) found in Nevada and neigh- 
boring states kills the sheep that eat it. The spongy leaves of this 
weed are filled with oxalic acid, which combines with the calcium 
in the blood and causes death within a few hours. 



Exploitation 385 

Many other instances of poisoning are known, some of them acting 
in a highly specific manner. The harm done in this way by one 
species to another may be completely fortuitous in some instances, but 
it may have evolved as a protective adaptation in others. The ecolog- 
ical consequences of this type of antagonism, involving the produc- 
tion of poisons or of harmful metabolites, have been discussed in fur- 
ther detail by Lucas (1949). 

Exploitation 

We shall now turn to relations of exploitation in which the mem- 
bers of one species benefit by the utilization of another speciesX The 
most common manifestation of interspecific exploitation is the use of 
a neighboring species as a source of food^nd this will be discussed 
tinder the headings of parasitism and predation. However, an or- 
ganism may employ another species of plant or animal in the environ- 
ment for attachment, support, or transport, as already mentioned, and 
such activity often results in harmful consequences to the host. 

Certain highly specialized kinds of exploitation occur, and one of 
the most astonishing of these is the enslavement of one species of ant 
by another^Talbot and Kennedy, 1940). Polyergus is an obligate 
slave-making ant that is unable to maintain its colony without the 
presence of members of certain species of Formica. Polijergm work- 
ers raid neighboring nests of Formica and carry home larvae and 
pupae. After maturing into the worker stage, these captive ants 
undertake feeding and nest building for their masters. Pohjergus 
ants will starve even in the presence of abundant food if this slave 
labor is not available. ^ 

Another special type of exploitation is that practiced by so-called 
"parasitic birds/' The cuckoo in Europe and the cowbird in the 
United States never build nests of their own; each female lays an 
egg in the nest established by birds of another, usually smaller, 
species. The cowbirds' habit perhaps arose as a result of their 
nomadic wandering after grazing animals, which left them insufficient 
time in one place for such domestic matters as raising a family. The 
pair of birds thus imposed upon is sometimes able to throw out the 
foisted egg or to cover it by building a new nest bottom over it, but 
more frequently the unwanted egg hatches out along with the host's 
own eggs. The young cuckoo soon pushes the other fledglings out 
of the nest, and the young cowbird is able to starve out its nest mates 
by grabbing all the food. After the young imposter has left the nest, 
its squawking still calls forth in its foster parents the behavior pattern 



386 Relations between Species 

of feeding and caring for it even in the absence of the rightful young. 
The spectacle of a small sparrow or warbler frantically trying to satisfy 
the hunger of an adolescent cowbird perhaps four times its size is an 
amazing sight. 

The "parasitic" activities of other kinds of birds are really in the 
nature of "hold-ups." The reader may have witnessed the spectac- 
ular sight of an eagle attacking an osprey high in the air, forcing it 
to drop a fish from its talons, and then catching the fish before it 
reaches the ground. Similarly, a skua or a jaeger will chase a gull or 
a fulmar until the pursued bird drops its fish, or disgorges a meal al- 
ready swallowed, and then the bully secures the food for itself. 
Various other kinds of animals are professional highwaymen, includ- 
ing some that compound the felony by robbing thieves. Such hi- 
jacking is carried on by certain flies (Bengalia) that waylay ants re- 
turning from a raid on a termite nest. The fly attacks an ant carry- 
ing away a termite, causing it to drop its booty, whereupon the fly 
quickly consumes the stolen goods. 

Parasitism. Strictly speaking, a parasite is an organism that 
resides on or in the body of a larger living organism and derives 
nourishment from its tissues. ^Accordingly, the foregoing instances 
of exploitation are not regarded as parasitism in the more precise 
meaning of the term, either because nourishment is not involved or 
because one organism does not live in contact with the other. * / Thus 
barnacles growing on the back of a whale, as well as other kinds of 
epizoans and epiphytes, are not true parasites since they do not eat 
the host's tissue. However, the limits of parasitism even in the strict 
sense are not sharp. Just as gradations exist between parasitism, 
commensalism, and symbiosis, so also many borderline situations ex- 
ist between parasitism and predation. Some organisms derive only 
a part of their nourishment from their hosts and some are in contact 
with their hosts for only a part of their lives. The typical parasite 
lives in its host without causing its death, and the typical predator 
kills the prey upon which it feeds. Yet some parasites regularly kill 
their hosts, and some organisms classified as predators eat only a part 
of their prey sometimes without causing significant harm. Generally 
speaking in parasitism the weak benefit at the expense of the strong, 
whereas in predation the relations are reversed and the strong exploit 
the weak. 

Examples of partial parasitism are found among various kinds of 
plants and animals. The mistletoes (Loranthaceae) grow like little 
shrubs on the branches of trees. Their specialized roots penetrate 
the vascular tissues of the host whence they obtain water with the 



Parasitism 387 

contained dissolved minerals. Mistletoe plants may be definitely in- 
jurious to the tree on which they grow, but, since they are abundantly 
supplied with chlorophyll, they photosynthesize their own carbo- 
hydrates. Blood-sucking bugs, flies, and leeches and vampire bats 
may be in contact with their hosts for only short period?, and, al- 
though such animals are commonly regarded as parasites^heir mode 
of life is clearly on the borderline between parasitism and predation. 
Little basis exists for considering mosquitoes, which live on the blood 
of animals, to be parasites and not so considering aphids, which suck 
the juices of leaves. But if aphids are regarded as parasites, their use 
of the material of a living plant is no different in principle from that 
of caterpillars which eat the leaf substance from the outside as herba- 
ceous predatorsor from that of deer which browse on the same leaves 
from the ground. Many partial parasites can derive nourishment 
from a wide variety of host species. These considerations bring us 
to the conclusion that every gradation exists between obligate para- 
sitism at one extreme, in which the parasite is completely dependent 
upon a living host with which it is permanently associated, and preda- 
tion at the other extreme, in which a free-living predator catches, 
kills and devours its prey. 

Many kinds of plants and animals have taken up a completely para- 
sitic mode of existence. Although the largest representation comes 
from the lower organisms, certain members of more advanced groups 
have also resorted to parasitism. Plant parasites, mostly fungi and 
bacteria, may' attack animals or other plants. Animals that parasitize 
other animals are found in the Protozoa, in various other invertebrate 
groups, and rarely in vertebrates. Animals that parasitize plants are 
represented by gall wasps and gnats. The eggs of these insects are 
commonly laid on stems and leaves, and the activities of the young 
cause the formation of galls on the plant host. No organism is known 
that is not susceptible to attack by parasites of some sort. 

Some parasites are restricted to one host species, or to one type 
of host, whereas other parasites can attack widely different plants and 
animals. Parasitism may even occur within a species. For example, 
in the deep-sea angler fish, Photocorynus spiniceps the male lives as 
a tiny permanent parasite upon the head or side of the female and ob- 
tains his entire nourishment from her blood supply. Thus does this 
species solve the problem of the location of one sex by the other in 
the inky blackness of the deep sea. This type of intraspecific relation 
occurs frequently in the plant kingdom, as seen, for example, in the 
growth of the pollen tube representing the male plant as a "parasite" 
on the tissues of the stigma and style of the flower of an angiosperm, or 



388 Relations between Species 

again in the growth of the sporophyte of a moss upon the tissues of 
the leafy gametophyte. In this chapter, however, we are concerned 
primarily with the parasitism of one species on another species. 

Great variety is displayed among parasites in regard to their loca- 
tion on their hosts, the duration of their dependency, their adapta- 
tions, and their methods of transmission. Only brief mention of the 
more ecological aspects of this large subject can be made here. For 
a more thorough discussion of parasitism the reader should consult 
such authors as Pearse (1942), Chandler (1944), or Baer (1951). 
Parasites may occur on the outside of the hosts (ectoparasites) or 
within their cavities or tissues (endoparasites). Certain worms and 
many bacteria and protozoans are obligate parasites practically 
throughout their lives; other organisms may be parasitic for only a 
small part of their existence. Facultative parasitism occurs widely 
and is found, for example, in mosquitoes that can live their whole 
lives on nectar and other plant exudates but will readily parasitize 
any available mammal. However, some species of mosquito must 
obtain a blood meal before they can reproduce. 

Many part-time parasites are parasitic as adults and free living as 
larvae, but in other species the reverse is true. A rather special con- 
dition within the latter category is presented by the parasitic wasps 
that lay their eggs in the bodies of other insects. When the eggs 
hatch, the larvae eat the tissue of the host, which at first remains 
active. Since the host is always killed eventually by the growing 
stowaway, the term parasitoid is sometimes applied to insects follow- 
ing this life pattern. Certain of the true wasps complicate the pro- 
cedure further by stinging their victim into permanent paralysis be- 
fore depositing the egg on it. In this way, the parent wasp provides 
a living but helpless insect of another species, or a spider, for the 
nutrition of the larva whose mode of life is thus transitional between 
parasitism and predation. Perhaps the most extraordinary example 
of this behavior is the attack of digger wasps, the largest of which 
have a wing span of about 10 cm, upon the even larger tarantulas. 
When the female of the giant wasp Pepsis rnarginata is ready for egg 
laying, she somehow locates a tarantula Crijtopholis portoricae, and 
explores it with her antennae to make sure that it is the correct species. 
The larvae of each species of wasp can be nourished by only one 
species of tarantula. Although the tarantula could easily kill the 
wasp, it does not do so, and makes little attempt to escape. After the 
wasp has dug a grave for its intended victim, she stings it (Fig. 10.11 ), 
drags it into the grave, and lays a single egg, which she attaches to 
the abdomen of the paralyzed monster. At hatching the wasp larva 



Parasitism 389 

is only a tiny fraction of the bulk of the tarantula, but, by the time 
it is ready for metamorphosis and independent life, it has consumed 
all the soft tissue of the giant spider ( Petrunkevitch, 1952). 




FIG. 10.11. The giant wasp (Pepsis marginata) stinging the tarantula (Cnjto- 
pholis portoricae) preparatory to attaching an egg to its abdomen. (Petrunke- 
vitch, 1952, Scientific American, drawing by R. Freund. ) 

Plants and animals are susceptible to invasion by several to many 
species of parasites at the same time, and conversely many parasites 
can or must have more than one type of host during their lives. The 
fact that parasites infest other parasites has been immortalized by the 
jingle: 

Fleas have lesser fleas 
Upon their backs to bite 'em, 
And lesser fleas still lesser fleas 
And so ad infinitum. 

In a chickadee nest two cowbirds were found both of which were in- 
fested with hippoboscid flies. Attached to the abdomen of one of 
the flies were two mallophagan bird lice that thus obtain transporta- 
tion from one bird to another, and within the bodies of the lice bac- 
teria were undoubtedly present (Herman, 1937). As many as five 
links in chains of such hyperparasites and symbionts have been re- 
ported. 

During the perpetual war between parasites and their hosts many 
special adaptations have evolved on both sides. The anti-invasion 
tactics of the host include external anatomical features and internal 



390 Relations between Species 

defenses of antibodies and phagocytes. The parasite is usually pro- 
vided with a protective covering and with special ways for gaining 
entrance, maintaining position, and avoiding digestion by the host. 
On the other hand, certain of the parasite's unused organs have de- 
generated. The efficient parasite gains all the advantage possible 
without seriously curtailing the life of the host. If the host is killed, 
or when it eventually dies, the parasite must have the means of reach- 
ing a new host. Both host and parasite may become so profoundly 
modified as to be almost unrecognizable. In its young free-swimming 
stage the barnacle Sacculina has the appearance of a typical crustacean 
larva, but, as an adult parasitizing the shore crab, it becomes little 
more than a sac suitable for the absorption of food and for reproduc- 
tion. The crab host is in turn reciprocally affected by the presence of 
the parasite for its reproductive glands are caused to atrophy and cer- 
tain metabolic changes are brought about. 

In many instances the transmission of the parasite to new hosts is 
left to chance aided by a very high natality; in others the parasite 
is specifically oriented to the host (Thorpe and Jones, 1937) or the 
host is attracted to the parasite. A remarkable instance of the latter 
procedure is displayed by the fresh-water mussel Lampsilis ventricosa 
in which the mantle edge is modified to appear like a small fish. 
When a real fish, attracted by this mimic, swims over the mussel, cast- 
ing a shadow, the mussel discharges its glochidial larvae. Some of 
these larvae reach the gills or fins of the fish to which they attach and 
live as parasites until they are ready to metamorphose into adults. 
Certain fishes thus parasitized wander upstream where the young 
mussels drop off and begin a new life as independent bottom ani- 
mals. In this way these sessile forms are distributed against the 
current to the upper reaches of the stream. 

Many parasites require more than one host for the completion of their 
life cycles; a large number of instances could be cited for both plants and 
animals. One familiar example is the white pine rust that is de- 
pendent upon members of the genus Ribes as a secondary host. The 
bass tapeworm that causes stunting and sterility in the small-mouthed 
bass also illustrates the complexities of multiple parasitism. In the 
spring when the bass swims into the. shoal water of a lake for spawn- 
ing, segments of the tapeworm (1 in Fig. 10.12) living in the fish's 
gut are discharged into the water where they produce eggs ( 2 ) . The 
eggs are eaten by copepods (3), a secondary host, and hatch out 
within the alimentary tract of these primitive crustaceans. The re- 
sulting larvae pass through the wall of the intestines into the body 
cavity of this host. Meanwhile young bass and other fish (4) 



Parasitism 



391 



we;* 




FIG. 10.12. Life cycle of the bass tapeworm (Proteocephalus ambloplitis, in- 
volving adult small-mouthed bass Microptcrus dolomieu, the copepod Cyclops, and 
young bass or other small fish. (Hunter and Hunter, 1929, New York State 

Conservation Dcpt. ) 



392 Relations between Species 

spawned in the same shore area have grown to the feeding stage and 
eat the copepods harboring the larvae of the tapeworm, A close cor- 
relation is thus required between the time for the copepod popula- 
tion to develop, for the larvae of the parasite to hatch, and for the 
young fish to appear. The larvae taken in with the copepods can 
resist being digested, as can the other stages of the parasite, and make 
their way into the body cavity, liver, or other organs of the small 
fish which is a tertiary host. When the infected young fishes are 
eaten by the voracious larger bass (5), the tapeworm larvae are 
passed on, and this transfer from smaller fish to larger fish may take 
place several times. The larvae do not develop into adults while in 
the body cavity or internal glands of the small fish. Only if the in- 
fected fish is eaten by a large bass at the moment when the larvae 
are ready to metamorphose, do they develop into adult tapeworms. 
This final stage establishes itself in the intestine of the primary host 
and thus completes the life cycle. In another species of fish tape- 
worm the adult stage is found in man and in other fish-eating mam- 
mals. The life cycles of flukes that parasitize birds, snails, and fish 
successively are described by Hunter (1942). These multiple rela- 
tions involve many special adaptations of the parasites and of the 
species in the environment on which they depend. 

Predation. The typical predator is free living and catches, kills, 
and devours individuals of another species for food, in contrast to the 
typical parasite that lives on or in its host and derives nourishment 
from it without killing it.y Thus the predator is said to live on "capi- 
tal" whereas the parasite lives on "interest." ^/^Ithough this concept 
is generally true as far as the individual aggressors and victims are 
concerned, it does not necessarily apply to the populations as a whole 
Parasites may kill a sufficient number of their hosts so that their "capi- 
tal stock" is very greatly reduced. For example, in the grouse dis- 
ease, which strikes this game bird periodically, the host population is 
so depleted by the ravages of the infectious parasite that the parasite 
population in turn sinks to a low ebb. Similarly, in an area in which 
the predator and prey populations have struck more or less of a bal- 
ance we may find that the predators are limiting themselves to "in- 
terest" in the sense that they are devouring only the increment to the 
prey population each year. In such a situation the predator popula- 
tion may continue indefinitely to take a limited number of the prey 
without endangering the breeding stock of the species on which it 
depends. 

Predatory organisms are almost entirely animals but a few kinds 



Predation 393 

of plants are also included. Predatory animals that eat other animals 
are carnivores, and those that feed on plants are herbivores. Some 
herbivorous animals kill the plants on which they feed by consuming 
all or most of each individual.^Aquatic filter feeders necessarily de- 
stroy all the diatoms, flagellates, and other algae that they digest, 
and herbivorous animals on land that feed on small plants may do the 
same. However, many insects and many ruminants browse lightly 
over the vegetation in such a way as to allow the plants to continue 
life indefinitely. We have seen that the grazing of sheep, for ex- 
ample may trim the grass harmlessly and prevent the invasion of 
other plants, thus aiding in the development of a healthy permanent 
turf. 

Usually plants serve as food for animals, but in a very few excep- 
tional instances the tables are turned, and animals fall prey to car- 
nivorous plants. ^ln the terrestrial flora these are also known as in- 
sectivorous plants since insects are the usual prey on land. Plants 
with this food habit are adapted in various remarkable and intriguing 
ways to attract, catch, and digest their victims. The sundew 
' Drosera ) for example, has round, reddish leaves provided with hairs 
that are progressively longer toward the periphery and tipped with 
glistening drops of a sticky, sweet secretion. When an insect, at- 
tracted by the color or odor of the plant, alights on the leaf, it sticks 
fast to this little bit of natural fly paper. The presence of the insect 
causes the hairs to bend over it; a digestive secretion is produced, and 
the body of the insect is absorbed>/The vase-like leaves of the 
pitcher plant are filled with water and serve as death traps for other 
insects (Fig. 10.13). Animals that fall into the pitchers are unable 
to climb out because of the steep sides and downward directed hairs; 
they eventually drown and are digested by special tissues at the base 
of the leaf. Further interspecific relations result from the fact that 
several protozoans and aquatic insects, including certain mosquito 
larvae, normally live in the water of the pitchers. In the larger 
plants of the tropics frogs station themselves at the openings of the 
pitchers and spiders spin webs across them to secure their take of 
the insects that stray past. 

Some plants are hunters of even smaller game. Specially adapted 
bladders of the aquatic bladderwort Utricularia open suddenly wher 
any plankton organism comes in contact with the trigger bristles 
the unlucky plankter is sucked inside and digested. Equally remark- 
able are adaptations that permit certain predaceous fungi to capture 
and to consume prey even larger than themselves. One species pro- 



394 



Relations between Species 



duces a chain of cells in the form of a lasso in which a portion of a 
nematode, or other soil microorganism, becomes ensnared and which 
then tightens up sufficiently to kill the victim ( Bessey, 1950 ) . 

Some predators are restricted to one prey species or are dependent 
upon a small group of food species, whereas others are highly catholic 
in their tastes, as has already been discussed in Chapter 8. Con- 
versely, certain prey species are attacked by only one predator, but 
others may satisfy the appetites of many kinds of diners. The special 
anatomical and physiological adaptations of predator species for 
securing, devouring, and digesting specific prey species, and the 
equally elaborate specializations of the prey to avoid detection or to 
resist capture, have been considered in earlier chapters. 




I'twto by H. W . Ail red, U. S. Soil (.Conservation Service 

FIG. 10.13. A group of pitcher plants (Sarracenia flava) in swampy land of 

Louisiana. 

In some animals only the adults are predatory, whereas the young 
are parasitic or live wholly upon the yolk supplied in the eggs. In 
other animals, such as many insects, the larvae do most of the eating, 
and in certain species the adults do not feed at all. Some predators 
feed upon the adult stage of their prey, some on the larvae, some on 
the eggs, and some on more than one stage. If members of the prey 
species are killed before they have had a chance to reproduce, the 



Predation 395 

inroads of the predators on the population will be particularly serious. 
The degree to which food "preferences" influence the feeding of 
predators is of considerable importance, both theoretically and prac- 
tically in relation to pest control, game management, and fishery 
biology. The forage ratio of a prey species (A) in respect to a 
predator species (B) is defined as the ratio of the percentage of B's 
food made up by A to the percentage of the potential food organisms 
in the environment represented by A: 

. %A in /Ts food 

Horace ratio = 7^7 /-. r 

%A in environment 

A forage ratio of 1 indicates that the species is eaten in the same pro- 
portion as its abundance in the habitat. An investigation of the 
feeding of the black-nose dace (Rhinichthys atratulus atratulus], for 
example, revealed a forage ratio of 2.7 for Diptera and of 0.47 for 
Trichoptera although the latter were about one quarter more abun- 
dant in the stream. In this instance the Diptera were found to be of 
size and in positions that rendered them more accessible to the fish 
than were the Trichoptera (Hess and Swartz, 1940). 

Generally speaking, predators tend to eat the organisms that are 
most available either because of their abundance or because of their 
accessibility. If a predator has a specific food preference or becomes 
conditioned to continue feeding on the same species, serious conse- 
quences may ensue both for the predator and the prey; but usually 
when one type of food becomes scarce, the predator changes its diet. 
Top minnows (Gambusia) are often stocked in reservoirs because 
fishes of this type are known to eat mosquito larvae voraciously. Un- 
fortunately for the mosquito-pestered human residents, when the fish 
have reduced the larvae to low numbers, they turn to other food and 
thus allow the mosquito population to recover. 

Studies by N. Tinbergen showed that during one winter, within a 
certain locality, when voles (Microtus) were abundant, they consti- 
tuted 86% of the food of long-eared owls (Asio otus) and wood mice 
(Apodemus) furnished 1%\ whereas in the following winter when voles 
were scarce, they formed only 30% of the owls' food, wood mice fur- 
nished 153S, and other animals made up the difference. A more elabo- 
rate investigation of the forage ratios of the sparrow hawk by L. 
Tinbergen has been reviewed by Hartley (1947). These quantita- 
tive aspects of prey-predator relations will be considered further in the 
section on competition and in subsequent chapters dealing with the 
interdependences of the community as a whole. 



396 Relations between Species 

Competition 

In the foregoing sections we have discussed certain categories of 
antagonism in which one species harms another by parasitizing it, 
by preying upon it, by poisoning it, or by taking some other direct 
advantage of it. In this section we shall consider first the aspect of 
competition that involves the "cold war" between species that are 
contending for "lebensraurn" or for "consumer goods," and then we 
shall discuss certain generalities in regard to competition involving 
both direct aggression and indirect rivalry. An organism competes 
with members of its own species as well as with representatives of 
other species for space, light, food, or other necessity, but the nature 
of the competition between species differs because of the variation 
in precise needs and adaptations of different species.,/ 

Lichens compete with each other for space on a dry ledge and also 
with members of other species; on a submerged rock barnacles simi- 
larly compete for space with one another and also with oysters, mus- 
sels, and other sessile animals. Tree seedlings vie with small shrubs 
and herbs for light in the developing vegetation. The roots of a 
forest tree engage in a continuing but unseen struggle with the roots 
of other trees for water and for nutrients. Various species of para- 
sites contend for the choicest tissues of their host. Grasshoppers not 
only compete closely with other insects for grass but also contend to 
some extent with mice and rabbits as well as with sheep and antelope 
for the same food. Various carnivorous species similarly are often 
rivals for the same prey. 

The more closely similar one organism is to another the more nearly 
alike will be their needs and hence the more intense will be their 
rivalry in obtaining their requirements from a common environment. 
This fact means not only that competition between individuals of the 
same species is particularly keen but also that the intensity of com- 
petition between species is directly related to the ecological similarity 
between them. The rivalry between species of the same genus is 
therefore usually more severe than that between species belonging to 
different genera as pointed out long ago by Darwin. 

Various aspects of interspecific competition have been investigated 
with carefully controlled laboratory populations of flour beetles. 
When Tribolium confusum and Tribolium castaneum, for example, 
are grown in the same "universe" of flour, one species always becomes 
extinct, leaving the other in sole possession, although plenty of food 
is available for both. The two beetles are very closely similar in life 



Competition 397 

history, behavior, and requirements, but subtle differences exist in the 
responses to the conditioning of the medium, in the effects of crowd- 
ing on natality, mortality, and rate of development, or in the rates at 
which the beetles eat each other's eggs. Under certain conditions 
T. confustwi was found to inhibit the net fecundity of T. castaneum 
more than the latter species inhibits itself, with the result that T. 
confusum drove out T. castancum in competition. Under slightly 
different circumstances T. castaneum gained the upper hand, but 
never would the two species coexist permanently in the same unit of 
flour (Birch, Park, and Frank, 1951). 

In similar tests involving mixed cultures of two species of clado- 
cerans, Daphnia pulicaria always caused the extinction of Sirno- 
cephalus vetulus. Since these species do not directly attack one an- 
other at any stage, the displacement of one species must be due to 
difference in toleration for shortage of food, chemical conditioning, 
oxygen lack, or physical effects of crowding, but precisely what is the 
crucial aspect of the competition awaits further investigation (Frank, 
1952). 

From both laboratory and field studies we learn that two species 
having essentially the same requirements from their immediate en- 
vironment do not usually form mixed steady-state populations. The 
most closely related species of a genus generally have different geo- 
graphical ranges. If they live in the same region, they inhabit dif- 
ferent types of habitat or they obtain their food and other necessities 
in a slightly different way in other words, they occupy different 
habitat niches, as discussed more fully in Chapter 13. 

When two closely competing species are unable to continue living 
in the same habitat, the determination of the species destined to 
survive depends upon which species is favored by the existing en- 
vironmental conditions. If conditions change, as they do regularly 
with the seasons in temperate regions, for example, a different species 
may come to be favored. When conditions are not optimal in re- 
spect to one ecological factor, the range of tolerance is often reduced 
with respect to other factors. These general relations help to explain 
the fact that different species succeed one another during the season, 
as is seen in phytoplankton populations, with no one species holding 
numerical superiority throughout the year. 

The type of competition between species that vie with one another 
by direct aggression has been given mathematical formulation by 
Volterra ( 1931 ) and Lotka ( 1934 ) for various cases in which one or 
more species feeds upon another species. When the population of 
one species grows at the expense of another, as in the predator-prey 



398 Relations between Species 

relation, the populations of the two species have certain reciprocal 
relations. These may be illustrated by the well-known investigations 
of Cause (1935) in which the interactions were demonstrated in 
laboratory populations of protozoans (Fig. 10.14). Cause prepared 
cultures in which the ciliate Paramecium caudatwn, as prey, was 
eaten by another ciliate Didinium nasutum, as predator. An ample 




Predator 



Predator 



III 




Time 

FIG. 10.14. History of prey (Paramecium caudatum) and predator (Didinium 
nasutum) populations in (I) homogeneous microcosm with initial seeding, (II) 
heterogenous microcosm providing refuge for prey with initial seeding, and (III) 
homogeneous microcosm with repeated seedings. (From Alice et al., 1949, after 

Cause. ) 

bacterial population was present at all times as a source of food for 
the paramecia. In the first type of experiment Paramecium and 
Didinium were introduced into a medium equally available to both; 
in the second type the microcosm was heterogeneous so that the 
Paramecium could escape into one part of the medium inaccessible to 
its aggressor. A homogeneous microcosm was again used in the third 
type of experiment, but additional seedings of both species were in- 



Competition 399 

troduced at regular intervals. In the first ?et of tests the prey popula- 
tion at first grew rapidly, but, as the number of predators increased, 
the prey gradually became extinct and thus caused the starvation of 
the predators soon afterward (Fig. 10.14, Case I). In the second ex- 
periments ( Case II ) since the prey could escape, their population con- 
tinued to grow, but the predators died off. In the third set of tests 
(Case III), in which the additional seedings simulated immigration, 
oscillations of both populations were induced with the maxima and 
minima of the predator population following those of the prey 
population. 

Many illustrations of the interdependencies of prey and predator 
shown in the foregoing tests are known in natural populations. The 
removal of predators has allowed certain useful fish populations to in- 
crease several fold (Huntsman, 1938; Foerster and Ricker, 1941). 
The protection of deer from predation in the Kaibab forest resulted 
first in their great increase and then in wholesale death from starva- 
tion, as discussed in an earlier chapter (Leopold, 1943). In most 
natural situations the predators are unable to find and kill all the 
prey so that, after the reduction of the predator population, the prf^ 
population can increase once more. Furthermore, since in nature 
not all the prey are successful in finding refuge from their enemies, a 
small part of the predator population usually manages to survive 
periods of prey scarcity. 

The action of these reciprocating influences may cause oscillations 
in natural populations of prey and predator even in the absence of 
periodic immigration. For example, in the Hudson Bay watershed 
the populations of lynx and of its prey, the varying hare, fluctuate 
widely and with about the same period of approximately 9% years 
(MacLulich, 1937). From the fact that the maxima and minima in 
the lynx population follow closely those of the hare population, as 
well as from other observations, the periodicity of lynx abundance 
appears definitely to be controlled by the availability of its food, and 
the oscillations of the hare population are influenced to some extent, 
at least, by the extent of lynx predation. Host and parasite popula- 
tions similarly affect one another in a generally reciprocal manner 
(Debach and Smith, 1947, and Utida, 1950). The repercussions of 
prey-predator and host-parasite fluctuations on the whole community 
will be considered further in Chapter 12. 

To summarize, when two antagonistic species occur in the same 
environment, these general types of result are possible: (1) If the 
two species are ecological equivalents making common demand on 
the environment, and if one inhibits the growth of the other more 



400 Relations between Species 

than its own growth, the first species will cause the elimination of 
the second from the area. (2) If the two species have somewhat 
different demands on the environment so that they inhabit different 
niches, the two species may continue to coexist in the area. (3) If 
the two species are dependent one upon the other, as parasite and 
host or predator and prey, the aggressor may eliminate the other 
species and then turn to other types of food. (4) If the attacking 
species is unable to destroy all the other species and is able to survive 
periods of low abundance of the other species, the two species may 
continue to live in the area in either a steady or a fluctuating equi- 
librium. For a further discussion of these intricate relations of com- 
petition the reader may consult Hutchinson and Deevey (1939), 
Solomon (1949), or Allee et al. (1949, Ch. 22). 

The survey of interrelations that has been presented in this chapter 
has revealed the extraordinarily varied and complex nature of the 
dealings of one species with another. We have delineated relations 
of mutual assistance, of toleration, and of antagonism; we have re- 
viewed the special circumstances of parasitism and of predation and 
have considered various ecological effects of competition between 
specific types of organisms. In the natural community many indi- 
viduals of many species of both plants and animals are typically 
present. It is our next task to consider the operation, organization, 
and alteration of the community as a whole in which each individual 
is responding not only to influences from all neighboring species but 
also to concomitant influences from others of its own species and from 
the chemical and physical features of the environment. 



Community Dominance 409 

example in an alpine region, or is largely unmodifiable as in the open 
ocean. This type of condition is also seen in communities of marine- 
bottom organisms in which the chief controlling influences are the 
mud of the bottom and the water currents. These factors act on the 
species present more or less directly, as indicated by the accompany- 
ing scheme (case 1). 

Mud-Bottom Biocenose 
/ / / / Water currents 

Case (1) A B C B 

I t t t 

- Substratum 



Case (2) A -**B *-(>* B 



Case (3) A +B *-C-<- B 

(Q (C) (C) 

In addition to the major effect of the physical features of the en- 
vironment a certain amount of interdependency may also exist among 
the members of biocenoses of this sort. In communities living on 
isolated rocks the interrelations between the species may be quite 
minor or entirely absent. When interdependencies do occur, the 
relations may be specific or non-specific. In case (2) species C is 
primarily controlled by the physical features of the environment, but 
it is also influenced by the presence of A and B specifically. An illus- 
tration of this situation is furnished by the fresh-water mussels that 
make use of fish swimming past as transport agents for their larvae. 
During most of their lives the primary dependencies of the mussels 
are concerned with the bottom, the water, and the plankton of their 
biotope, but at the time of reproduction this particular relation with 
the fish of their community becomes significant. 

In a third type of situation the presence of the other species exerts a 
definite effect but not in a specific way. In other words, as far as C 
is concerned, A and B may merely be occupying space, and individ- 
uals of C could be substituted for them without significant conse- 
quence to C as is suggested in case ( 3 ) above. Illustrations are found 
in the competition for space among attached animals on submerged 
rocks. Similarly, in the competition for light among plants the 
proximity of other individuals often has its chief effect in cutting off 



410 The Community 

the light with little difference whether the interfering neighbors are 
of the same or of different species, 

In many communities one species is particularly conspicuous 
because it is the largest or the most numerous. This species is often 
called the dominant, and its name is usually given to the community, 
as in a spruce forest community. Occasionally two or more species 
share the honors in respect to dominance, as in an oak-hickory forest. 
Very frequently the species that is dominant in conspicuousness also 
appears to exert a controlling influence over the other members of the 
community, but sometimes a less conspicuous species may have a pre- 
ponderant influence on the inhabitants of the biotope. In a sphag- 
num-heath community, for example, the heath plants are the most 
conspicuous, but the presence of the sphagnum controls the nature of 
the community as a whole. In some biocenoses microorganisms may 
play the critical part although they are among the least conspicuous 
of the species present. The seedlings of pine and some other trees 
do not grow properly unless suitable fungi are present to form mycor- 
rhizae on their roots. Thus, although the species designated as the 
dominant frequently does exert the major controlling influence on 
the biocenose, this is not necessarily true even in those communities 
in which a high degree of interspecies dependence exists. 

In the second general type of situation, in which the members of 
the community owe their presence primarily to a more or less direct 
and independent control by physical features of the environment, 
a species designated as dominant because of prominence or abun- 
dance clearly does not exert a critical control over the other inhabit- 
ants. For this reason some ecologists refer to such a species as "pri- 
mary" rather than dominant. Dominance based on controlling in- 
fluence must be clearly distinguished from dominance based on mere 
conspicuousness. In communities of low interspecies dependence the 
relative numbers and prominence of the various species are likely to 
vary from place to place. Great care must be taken in applying 
names to such communities to avoid ambiguity and the implication 
that conspicuous species are necessarily controlling. 

Ecotone 

The transition zone of tension between communities presents a 
situation of special ecological interest and is known as an ecotone. 
The border between forest and grassland, the bank of a stream run- 
ning through a meadow, or the boundaries between any other com- 
munities on land or in the water furnish illustrations of ecotones 



Ecotone 



411 



(Fig. 11.4). In this transition zone of tension the outposts of each 
community are maintaining themselves in environments that are in- 
creasingly unfavorable. The tension may result chiefly from a strug- 
gle with physical conditions or from a direct competition between 
certain species in each community. At the border between a shrub 
community and a marsh, for instance, the shrubs may compete 
directly with marsh reeds for light, nutrients, or other necessities of 




FIG. 11.4. A distinct ecotone dominated by smooth sumac (Rhus glabra) and 
goldenrod (Solidago) between forest and grass communities in eastern Massachu- 
setts. 

life in such a way that one type of plant gives way completely to the 
other. In such a situation and in areas where controlling physical 
factors change rapidly the transition between communities is abrupt 
and the ecotone is correspondingly narrow. In other circumstances 
the two communities may interdigitate to a considerable extent. At 
the edge of a forest individual trees may pioneer into a scrub com- 
munity, and the scrub species will invade the margin of the forest as 
far as they are able to survive. 

When one community gives way only gradually to the other com- 
munity, a wide ecotone results/ The transition zones sometimes 
more than 100 km in width between major continental communities 
are regarded by some as ecotones (Pitelka, 1941), but others restrict 
the term to areas of smaller scale. Strictly speaking a transition 



412 The Community 

area is an ecotone only if tension exists between the bordering com- 
munities, and this is often difficult to demonstrate, especially for 
large areas. Accordingly, the decision as to what is to be considered 
an ecotone depends upon the scope of the biocenose as recognized by 
the individual ecologist. 

In the ecotone area the conditions of temperature, moisture, light, 
wind, and other physical influences are different from, and usually 
intermediate between, those existing well within either of the border- 
ing communities. These or other conditions, such as food or shelter, 
may be superior in the region of the ecotone for certain species. 
These special influences of the ecotone area prevail below ground and 
high in the vegetation as well as on the surface of the soil. As a 
consequence various kinds of plants and animals not occurring, or rela- 
tively rare, in the bordering communities may become abundant in 
the ecotone. Shrubs typically grow in profusion at the forest edge 
and harbor a distinct fauna. At the margin of a pond willows and 
cattails thrive in the transition between land and water, and here 
are found such animals as turtles, frogs, herons, red-winged black- 
birds, muskrats, and a host of invertebrates that are entirely absent 
or much less abundant in the center of the pond or in the terrain far 
removed from the water. 

These species favored by the special conditions of the ecotone join 
with the outpost representatives of the principal inhabitants of the 
bordering communities in populating the ecotone. The species oc- 
curring in the ecotone may thus form a distinct functional community 
of their own. It must be remembered, however, that the ecotone 
inhabitants owe their existence to the presence of the particular con- 
ditions on each side and that the ecotone assemblage would disappear, 
or be considerably modified, if the bordering communities or condi- 
tions were removed or seriously changed. 

As a rule the ecotone contains more species and often a denser 
population than either of the neighboring communities, and this gen- 
erality is known as the principle of edges. The greater variety of 
plants in the ecotone provides more cover and food, and thus a greater 
number of animals can be supported. In measurements of bird 
populations in a variety of areas in the central part of the United 
States Kendeigh (1944) and Johnson (1947) distinguished between 
"forest-edge" species and species which confine their territory, nest- 
ing, feeding, and roosting to the interior of the forest. 

As conservationists and wildlife managers have become aware of 
the principle of edges, they have bent their efforts toward increasing 
the amount of available ecotone area in each region since this usually 



Ecotone 

A: Poor interspersiond covey) 



413 



Cultivation 




B: Good interspersion (6 coveys) 




FIG. 11.5. Diagram indicating the increase in number of coveys of quail sup- 
ported by farmland after increase in the amount of interspersion of the vegetation 
without change in the total area of each. (Reproduced from Game Management 
by Aldo Leopold, Copyright 1933 by Charles Scribner's Sons, used by permission 

of the publishers.) 



414 The Community 

favors the increase in game birds and mammals. Regions that are 
broken up into many units of different vegetation types are said to 
have "good interspersion" because the total length of community edge 
is great and hence a large amount of ecotone habitat is provided. 
Since coveys of quail tend to establish themselves where several 
types of vegetation meet, the number of coveys supported by farmland 
can be increased through a change from a few big tracts of woods, 
brush, pasture, and fields to a greater number of smaller interspersed 
tracts without altering the total area of any one type of vegetation 
(Fig. 11.5). 

COMMUNITY COMPOSITION 

Communities inhabiting distinctive and commonly occurring bio- 
topes are the most easily recognized, and their composition and 
interrelations have been more extensively studied than others. A 
biocenose of this kind is made up of a characteristic, but flexible, 
assemblage of species without necessarily containing any species that 
is exclusive to it. Certain types of species invariably occur in a 
biocenose of a certain kind, but the individual species may vary 
widely from place to' place. In a prairie community grass-like plants 
are always present and serve as the fundamental plant producers, 
but in different regions quite different species of grass fill this niche. 
Among the animals of this community fleet browsing forms, jumping 
types, and burrowers are commonly present but the species, genera, 
or even families of these typical members of the biocenose may differ 
greatly, depending upon the region in which they are located. The 
ecological relationships are entirely separate from the taxonomic af- 
finities of the species since the latter depend upon the evolutionary 
history of the area. Species having a common evolutionary origin are 
placed in "faunal regions" or "floral regions," and these subdivisions 
may or may not correspond with ecological subdivisions. 

Communities may be large or small. Some may cover thousands 
of square kilometers, such as a spruce forest in Canada, a prairie 
community in central North America, a pine-land community in 
southeastern United States, or an oceanic community. Other biocen- 
oses, such as those occupying relatively uniform swamp, desert, or 
lake biotopes, may have dimensions in hundreds of kilometers. The 
biocenoses of ponds, tide flats, rivers, balds, alluvial sands, chaparral, 
mountain meadows, and rocky plateaux typically occupy a more re- 
stricted area. Smaller communities inhabit canyons, Tiammocks" in 
the everglades, mountain springs, tidal inlets, and clearings in the 
forest. The plants and animals living on an isolated boulder, or in a 



Community Composition 415 

rotten log, or in the debris washed up on the shore of a lake constitute 
still smaller biocenoses. Characteristic communities occupy even 
more minute inicrohubituts in the area such as a crevice in a ledge, 
or the water in a pitcher plant. We thus have communities within 
communities. The epiphytes, cpizoans, and parasites of an animal 
or a plant may also be thought of as comprising a biocenose. An 
apple tree and the plants and animals that live on it and in it illustrate 
a special community of this sort, A horseshoe crab with the dozen 
or so species of mollusks, worms, and barnacles attached to its shell 
exemplify a microcommunity, as do the fauna and flora of a cow's 
stomach. 

The number of species and the population abundance in communi- 
ties also vary greatly. While maintaining the necessary quantitative 
relationships among the consumers and the producers in the eco- 
system, the inhabitants of an area often include a wide range of densi- 
ties. Furthermore, each component in the ecosystem may be repre- 
sented by one, a few, or many species. Biotopes with extreme con- 
ditions and little food, such as a rock desert or an ocean deep, gen- 
erally support few species and relatively few individuals of each 
species. The relatively small number of both species and individuals 
in the badlands sections of southwestern United States is familiar to 
those who have crossed these areas. In other severe habitats suffi- 
cient energy and nutritive materials are sometimes available to allow 
huge populations to develop although only a few species are able to 
survive. In the relatively simple biocenose on Bear Island in the 
Arctic Zone some of the smaller plants and animals are very abun- 
dant (Elton, 1939). 

Under more equable climates larger numbers of species generally 
become interrelated in community groups; each species is usually 
rather meagerly represented, but sometimes the abundance of certain 
species is great. The food relations of the North Sea herring are 
known to involve a great number of species (Fig. 11.6) some rep- 
resented by very large populations and these food species form 
only a portion of the whole community of which the herring is a 
member. A bed of kelp (Nereocystis luetkeana) in Carmel Bay, 
California, was found by Andrews (1945) to form the matrix of a 
community that included 40 species of invertebrates, several kinds 
of fishes, and numerous attached algae, protozoans, and bacteria, as 
well as associated species of phytoplankton and zooplankton. 

Few complete enumerations have been made of both the plants and 
animals in even the simplest situations and none in complicated 
biotopes especially in those with elaborate populations of micro- 




416 



Community Composition 417 

organisms. The complex ecosystems of temperate and tropical ter- 
restrial areas are particularly difficult to analyze, but some idea of the 
number and diversity of the living components may be obtained from 
existing studies of portions of the biota. A comparison of the num- 
bers of species that may be involved in communities of different kinds 
has been made by Elton (1946); a few examples follow. 

The biocenoses of certain British rivers include representatives 
of as many as 131 species of invertebrates in addition to fishes, 
amphibians, rooted aquatic plants, and vast populations of algae, 
flagellates, and bacteria that were not enumerated. A meadow on clay 
near Oxford, England, served as the biotope for 93 species of inverte- 
brate animals in the soil and surface vegetation, in addition to the 
plants and the microorganisms. At the other numerical extreme, the 
biocenose on the sandy shore of Amerdloq Fjord, west Greenland, con- 
sisted of only 5 species of invertebrates. In the Bettila odorata forest 
in Finmark, Norway, only 29 species of plants were reported, whereas 
in an ash forest in Yorkshire, England, 72 species were found, and in 
the red fir forest of the Sierra Nevada, California, 93 species of plants 
were recorded. The animal components of these forest communities 
were not recorded, but they would include birds, mammals, am- 
phibians, and a great many species of insects and other invertebrates; 
the total number of species would be further swelled by a wide 
variety of protozoans and cryptogamic plants. Even within a single 
microhabitat a surprising number of animals may be present as evi- 
denced by counts of 111 species of invertebrates found in pine logs 
and 136 species found in oak logs during investigations in the Duke 
Forest, North Carolina. 

Another observation in regard to community composition is that 
the species present for the most part belong to different genera. In 
many biocenoses each genus is represented by only one species; in 
others certain genera are represented by several species, as is often 
true, for example, of the sedges, willows, and oaks in communities 
of temperate and boreal North America. Rarely are more than a few 
species of a genus found in the same biocenose. Quite exceptional 
is the occurrence of "species flocks," such as those in Lake Baikal, 
where, for example, 300 species of gammarid crustaceans are found 
and these belong to only 30 genera (Brooks, 1950). The typical 
biocenose is composed of species that are separated by at least generic 
differences, whereas in the biota of a country or a region many species 
may be recorded for each genus. Comprehensive surveys of the 
animals in 55 communities and of the plants in 27 communities from 
a wide range of habitats showed that 86 per cent of the animal genera 



418 The Community 

and 84 per cent of the plant genera were represented by only one 
species (Elton, 1946). The average number of species per genus 
in these widely spread surveys was 1.38 and 1.22 for the animals 
and plants, respectively. In contrast the average number of species 
per genus for all British insects, for example, is 4.23. 

As a general rule the species of a genus are sufficiently similar so 
that their demands on the environment are in serious conflict. During 
the course of evolution this conflict has usually resulted in eliminating 
all but one species from each genus represented in the area. In 
contrast to a geographical region, a community is a functional unit 
and each niche in the community structure is occupied by that species 
which has been most successful in the competition for it. Adaptations 
for ecological needs and accessibility determine which species will 
survive and populate each part of the biocenose. The species that 
succeed in establishing themselves are almost always sufficiently dif- 
ferent to be members of separate genera. Competition within each 
level of nourishment in the food chain thus produces a certain or- 
ganization among the species of a community in addition to the 
organization imposed by the relationships from one link in the food 
chain to another and by other dependencies. The species composi- 
tion of each biocenose is therefore determined by relations within 
the same functional level as well as in successive levels from among 
the species that are distributed to the area. 

To make a census of the inhabitants of a diversified natural com- 
munity and to ascertain the role of the various species in any compli- 
cated ecosystem is exceedingly difficult. Such tasks require the 
combined efforts of at least several taxonomists and ecologists com- 
petent to deal with the various segments of the biota. General ac- 
counts of the "plant communities" and the "animal communities" in- 
habiting the principal types of environment in the world are available 
in such works as Costing (1948), Hesse, Allee and Schmidt (1951), 
and Dice (1952). Detailed studies of the inhabitants of specific 
biotopes have for the most part been carried out by investigators con- 
fining themselves to restricted categories of animal or plant life. To 
obtain an idea of the total assemblage of organisms making up a 
biocenose and their integrated activities, it is usually necessary to 
piece together the results of several investigations carried out in the 
same region. The reader may obtain rather complete information 
on the composition of the entire biota of certain specific areas by 
reference to the collaborative reports of members of expeditions, of 
government surveys, and of field research laboratories. In many 
instances the work of areal surveys has been expanded to include 



MATURE POPUP COMMUNITY 



Baltimore oriole 
Chickadee 
Least flycatcher 
Rose-bstd grosbeak 
Willow thrush 



Canker 
Pomes 



ASPEN 
Dogwood 
Hazelnut 
Wintergreen 
Sarsaparilla 


* 


^^_ 



Hairy and Downy 
woodpeckers 

Dicera 
Saperda 
Ruffed grouse 

t \N 

Groshawk 



Cooper and 

Sharpshinned 

hawks 



.Rabbit 
Crow. 

V\ Great horned owl 



FOREST EDGE 
IMMATURE ASPEN COMMUNITY 



Yellow warbler 
Redwinged blackbird 
Bronze grackle 



ispen 
Show- 
berry 

Hazelnut 
Choke- 
cherry 



Red-eyed 

vireo 
Yellow 

warbler 
Gold finch 
Catbird 
Brown 
thrasher 
Townee 

robin 



Cutworms 
Grasshoppers 
Click beetles 



Pocket gophers 
Ground squirrels 




FIG. 11.7. Simplified representation of four communities in the parkland region 

of Manitoba, showing some of the food relations within and between communities. 

(Reprinted with permission from Hesse, Allee, and Schmidt, Ecological Animal 

Geography, 1951, John Wiley & Sons.) 

419 



420 The Community 

studies of the dynamic ecological relationships within the community. 
The extensive series of investigations on Wisconsin lakes will serve as 
an example (Juday, 1943). 

The foregoing discussion has shown that the biocenose is composed 
of species characteristic of the area, but these species may not be 
limited to the area nor is the biocenose limited to a fixed set of species. 
This concept of the flexibility of the biocenose is well illustrated by 
studies made by Bird (1930) of the interrelationships of members of 
communities occurring in the parkland region of Manitoba. The 
chief food relations of certain of the species within and between com- 
munities are indicated in Fig. 11.7. A great many other, smaller 
species are present in each community represented in the diagram, 
and many other dependencies exist besides the food relationships. 
This oversimplified picture nevertheless shows that certain kinds of 
plants and animals form a characteristic nucleus in each community 
but many of the individual species occur in more than one biocenose. 
The principal members of each biocenose have most of their require- 
ments fulfilled within their own biotope but interrelations between the 
biocenoses are brought about by animals that move from one biotope 
to another for foraging or other purposes. 

STRATIFICATION OF THE COMMUNITY 

Many communities exhibit a structure, or recognizable pattern, in 
the spatial arrangement of their members. A community may be 
divisible horizontally into "subcommunities" that is, units of homo- 
geneous life-form and ecological relation. The zonations described 
in earlier pages of this chapter are horizontal structural units of this 
sort. Of more general occurrence is the aspect of structure that in- 
volves vertical changes, or stratification, within the community. In 
some communities a complex stratification is present, but in others the 
vertical dimension is so much compressed that the entire biocenose 
consists essentially of only one stratum. Lichens pioneering on a 
rock ledge represent an extreme example of a one-layered community, 
but, at a later stage when higher-growing mosses and herbs have be- 
come established and bacteria are present in a layer of humus beneath 
the holophytic plants, the beginnings of stratification are apparent, 
albeit on a very small scale. 

In a grassland community subterranean, floor, and herbaceous sub- 
divisions can be recognized. The subterranean stratum contains the 
roots of the principal vegetation and forms the permanent residence 
of the soil bacteria, fungi, and protozoans, as well as a host of insects, 



Stratification of the Community 421 

spiders, worms, and other invertebrates. In addition, it provides a 
part-time abode for many other animals including other species of 
insects, rodents, reptiles, mammals, and a few burrowing birds. The 
main activity in the soil for both plants and animals occurs in the 
upper layers, but the longer roots of prairie grasses extend to depths 
of about 2 meters and the roots of other prairie plants occasionally 
reach levels of 5 or 6 meters below the surface. In an extensive 
discussion of the relation of underground plant parts Weaver and 
Clements (1938) report that 65 per cent of a group of true prairie 
species studied have root systems that penetrate to depths between 
1.5 and 6 m. Prairie ants are active to 3 m, and prairie dogs are 
known to burrow to more than 4 m. 

The floor subdivision of the grassland community contains basal 
portions of the vegetation, including particularly the rhizomes of the 
grass plants partially covered by litter and debris of both animal and 
plant origin. A characteristic group of animals in which insects, 
spiders, reptiles, and rodents are prominent join with the plants to 
form this subdivision of the community. Most of the animals inhabit- 
ing the grassland floor also usually invade one or both of the other 
subdivisions. The herbaceous stratum of this biocenose consists of 
the upper parts of the grasses and herbs and a characteristic assem- 
blage of animals. The vertical dimension of this subdivision is vari- 
able, according to the plant species and the local conditions. The 
area may be covered by sparse, resetted grass only a few centimeters 
in height or by coarse tough species growing 2 or even 3 m high. 
The animals of the herbaceous subdivision include a wide variety of 
insects, birds, and ruminants. 

Stratification of land communities reaches its greatest complexity in 
the forest. Five vertical subdivisions of the forest biocenose are 
typically present, namely: the (1) subterranean, (2) forest-floor, 
(3) herbaceous, (4) shrub, and (5) tree strata (Fig. 11.8). Each 
of these may exhibit certain further subdivisions in particular situa- 
tions, and the air above the forest canopy is sometimes considered to 
comprise a recognizable division of the community. Allee (1926) 
distinguished 8 strata in the tropical rain forest of Barro Colorado 
Island. The subterranean layer of the forest is typically damp and 
contains a large amount of humus that in extreme cases may be 
prominent to a depth of 2 or 3 m. The effective depth of the sub- 
terranean division of the forest community is difficult to determine 
but in wet forests it appears to be definitely shallower than in dry 
prairies. The adequate supply of water near the surface, the avail- 
ability of nutrients, and the inability of roots to penetrate into poorly 



422 The Community 

aerated soil influence the depth of root growth. In the temperate 
forest the tree roots may extend to depths of 3 m, but the bulk of 
root development occurs in the upper meter. Animal activity is most 
intense in the upper half meter but the larger burrowing forms pene- 
trate to depths of several meters. 




FIG. 11.8. Stratification of vegetation in a deciduous forest of the temperate 

region. The vertical subdivisions of the biocenose are: (1) subterranean, (2) 

forest-floor, (3) herbaceous, (4) shrub, and (5) tree strata. 

The forest floor is a complex subdivision of the biocenose in which 
great biological activity goes forward involving the decay of plant 
material and a web of predator-prey and parasite-host relationships. 
Huge numbers of invertebrates inhabit fallen logs and other material 
on the forest floor, and they are preyed upon by carnivorous beetles 
and hunting spiders as well as by higher animal types. Saprophytes, 
including the familiar mushrooms and bracket fungi, are particularly 
characteristic here but mosses and other low growing green plants 
may be abundant. In general the density of the population of the 
floor stratum in the forest is higher than it is in grassland and higher in 
warmer climates than in colder climates, 



Stratification of the Community 423 

The subdivisions of the forest biocenose above the ground are 
variable and are determined by the arrangement of the vegetation, 
but in most instances herbaceous, shrub, and tree strata are distin- 
guishable. The herbaceous stratum varies in height up to a meter or 
so and frequently overlaps with the shrubs that extend to heights of 
perhaps 1 to 5 m. The tree stratum occurs between heights of 5 and 
15 m in the typical oak forest, but it extends to 25 or 30 m in the 
coniferous forest and to about 40 m in the rain forests, with individual 
large-crowned trees towering to 50 m or more. The upper limit of 
the forest canopy in groves of giant redwoods may surpass 100 m. 
The herbaceous stratum is poorly represented or absent in some in- 
stances as, for example, under a thick stand of pines or spruce, and 
the shrub stratum may vary greatly in prominence. On the other 
hand, the tree stratum may be elaborately developed and divisible 
into sublayers. The trees of the rain forest on the Gold Coast of 
Africa display a stratification that extends from a height of 2 m or so 
to about 40 m (Foggie, 1947). 

The herbaceous, shrub, and tree strata are each inhabited by a 
characteristic assemblage of epiphytes and epizoans. Herbivorous 
insects and web spiders are particularly abundant in the herbaceous 
stratum. In the higher strata numerical superiority is held by the 
insect group in enormous variety but snails, lizards, snakes, frogs, 
arboreal mammals, and many kinds of birds are also present in greater 
or lesser abundance. Most of the species are primarily associated 
with one of the principal strataor perhaps with one of the subdivi- 
sions of the tree stratum but individuals of many of the species range 
above or below their usual abode. 

In certain aquatic habitats vertical gradients in environmental fac- 
tors cause a recognizable stratification among the members of the 
community. Such layering may have dimensions of less than a meter 
in shallow ponds, or it may involve strata many meters thick in the 
open ocean. Mention has been made in earlier chapters of the sharp 
limits commonly found for the vertical ranges of attached plants and 
animals in the littoral zone, and particularly in the tidal zone, as a 
result of the rapid change in water and light conditions. Where these 
sessile organisms exert a controlling influence on a variety of de- 
pendent species, they cause subdivisions of the community to be 
formed that may be directly observed (Stephenson and Stephenson, 
1949). 

The stratification of benthic communities at greater depths is not so 
easily recognized, but it is being investigated by means of dredges 
and underwater cameras, and by divers with aqualungs. The situa- 



424 The Community 

tion for communities in the free water contrasts with that for com- 
munities on the bottom or on land in the absence of members that are 
fixed in position. Since the pelagic plants and animals are free to 
drift or to swim vertically as well as horizontally, subdivisions of the 
pelagic community are less well defined and are more flexible than 
those determined by sessile algae or rooted vegetation. Nevertheless, 
critical changes in the physical environment, as at the lower limit of 
the photic zone, at the thermocline, at a density discontinuity, or at 
the margin of an oxygenless stratum bring about a definite stratifica- 
tion within many pelagic communities. 

Many investigations have been made in marine and fresh-water 
environments of the vertical distribution of various taxonomic groups 
among the plankton, fishes, and benthic organisms. Summaries may 
be found in such reference works as Welch (1952) and Sverdrup 
et al. (1942). However, relatively little study has been undertaken 
of the vertical subdivisions of aquatic communities as functional 
units. Certain zones of functional dependence are known to exist 
at various levels along the bottom and in the free water. One such 
unit of interdependence is formed by the plankton and the fishes of 
the well-illuminated surface waters, another is composed of the fishes 
and invertebrates inhabiting the bottom in deep water, and several 
others may be distinguished at intermediate levels. More intensive 
study will be required of the interdependencies among all the in- 
habitants of each stratum before specific functional subdivisions of 
pelagic communities can be clearly delineated. 

Although recognizable stratification does not exist in all communi- 
ties, its presence is sufficiently common in aquatic situations as well as 
in terrestrial areas for it to be considered a general characteristic of 
community structure. Spatial organization may thus be added to 
the other attributes of the community that have been reviewed in this 
chapter. Each community has been shown to have a composition 
and an integration among its members that separate it from neighbor- 
ing communities. The degree of dominance within the community 
and the nature of the ecotones between these units of plant and animal 
life vary greatly from one situation to another. All these charac- 
teristics allow the community to maintain itself as a recognizable unit 
in a specific area for a period of time. Circumstances that sooner or 
later may cause the replacement of one community by another or 
that may bring about fluctuations within the community will be con- 
sidered in the next chapter. 



12 

Succession 
and Fluctuation 



In the previous chapter the nature of the community was discussed 
as a characteristic group of plants and animals inhabiting an area. 
The typical community maintains itself more or less in equilibrium, 
but the members of the community are never in complete balance 
with each other or with the physical environment. Changes in the 
environment over a period of time are produced by variations in 
climatic and physiographic influences and also by the activities of the 
plant and animal inhabitants themselves. These modifications of the 
habitat may cause sufficient changes in the dominant species so that 
the existing community is replaced by a new community, or they may 
cause marked fluctuations in the abundance of certain species within 
the same community. In this chapter we shall consider the progres- 
sive changes in communities leading to a relatively stable type of 
community, the classification of communities, and the oscillations 
within the community. 

ECOLOGICAL SUCCESSION 

Progressive changes in communities take place from one geological 
epoch to another and also within much shorter periods of time. De- 
tailed consideration of large-scale changes in the fauna and flora, such 
as those caused by the passage of an ice age or the uplifting of a 
mountain range, or those resulting from the evolution of new species, 
is beyond the scope of our present task; such alterations in the biota 
have great long-term significance, and they are discussed in treat- 
ments on paleontology, climatology, biogeography, and evolution 
(Shapley, 1953). Here we shall concern ourselves primarily with 
the replacement of one community by another in particular areas and 
within the same general climatic conditions. 

Observation has revealed the fact that in given biotopes certain 

425 



426 Succession and Fluctuation 

communities tend to succeed one another. The occurrence of a rela- 
tively definite sequence of communities in an area is known as eco- 
logical succession. The change in the communities may be due in 
part to independent physiographic changes such as alteration of drain- 
age, erosion, or deposition, but more especially it is caused by modi- 
fications produced by the action of each community on its own en- 
vironment. The two types of causes are frequently operating to- 
gether, as is seen, for example, in the replacement of a pond com- 
munity by a marsh community. The filling of the pond is brought 
about by the deposition not only of a certain amount of inorganic 
silt, but also of a large amount of the organic remains of successive 
communities, and the accumulation of both types of deposit is en- 
hanced by the presence of the roots and stems of the living com- 
munity members. 

The extent to which ecological succession is self-induced as dis- 
tinct from being caused by changes imposed from without varies 
greatly in different situations. Similarly, the predictability of the 
course and speed of succession is variable. In many instances the 
presumed course of succession is based on inference derived from 
studies of surrounding areas so that "space is substituted for time"; 
but in other instances, some of which are described below, the nature 
of the succession is substantiated by actual records. Self-induced 
ecological succession is another outstanding example of the organism 
and the environment acting as a reciprocating system. 

Living things modify their own habitat so as to cause one com- 
munity to give way to another in a variety of ways. All species of 
animals and plants tend to increase in numbers and /or in size. The 
conditions of the community consequently change because of the 
growth of the inhabitants even without any change in species com- 
position. Consider a forest, for example. As the trees increase in 
size, they provide more shade, higher humidity, and different condi- 
tions of food and cover. New types of animals find suitable conditions 
here; old forms may be eliminated. Wildlife managers have come to 
realize that the carrying capacity of a forest area for game changes 
with time because the availability of food and shelter in a stand of 
saplings is entirely different from that in a stand of mature trees. 

When populations grow in respect to numbers or size of individuals, 
or both, the total weight of living material in the area tends to become 
larger. As predators and parasites increase in numbers, they tend to 
reduce the abundance of their prey, but as food becomes scarcer the 
consumers in turn are curtailed. At the same time the community 



Dispersal and Invasion 427 

causes changes in the physical nature of its biotope. Plants with- 
draw material from the soil as they grow and return it when they die, 
but the material returned to the soil is not in the same form. Humus 
accumulates, pH changes, moisture content is modified, and other 
alterations of the environment discussed in previous chapters are 
brought about. 

The changed conditions caused by the varied activities of the in- 
habitants of the area may favor the growth of species other than those 
that have been dominating the scene. When this occurs, different 
species will soon get the upper hand. These may either be species 
already present in a subordinate capacity, or invaders from the out- 
side. As one or more species take over the dominant position, a new 
community will be formed; its establishment constitutes a step in the 
ecological succession of the area. 

Dispersal and Invasion 

The establishment of the pioneer community on a bare area and 
the replacement of this community and of subsequent communities 
as ecological succession goes forward are dependent in the first in- 
stance upon the existence of means by which new species can reach 
the area. The ways in which animals and plants can invade new 
areas are extremely varied. Certain non-motile forms may be carried 
by wind or water currents for great distances; other species ride as 
hitchhikers on or in the bodies of various larger animals. The action 
of the air and water media in providing dispersal and the many special 
adaptations of eggs, seeds, and adults for transportation have been 
described in earlier chapters. 

The organism's own locomotion is responsible for the dispersal of 
many animals and of some motile plants. The success with which the 
starling extended its permanent breeding range westward in the 
United States from its point of introduction is indicated in Fig. 12.1. 
The great distances traveled by ducks after the breeding season and 
before undertaking the southward migration to winter grounds have 
already been mentioned (Fig. 9.19), Many other species probably 
wander radially from the breeding grounds in similar fashion before 
going to their winter quarters. Animals with such ample powers 
of dispersal can push rapidly into new areas suitable for their 
permanent invasion. Although other animals cannot travel at such 
speeds as those represented by the flight of birds, their incessant 
pressing against their boundaries nevertheless accomplishes wide 



428 



Succession and Fluctuation 



dispersal over a period of time. Even non-motile animals and plants 
are able to extend their ranges with surprising rapidity when condi- 
tions are suitable. 

Barriers. Against this insistant pressure for dispersal, barriers exist 
that retard or prevent the movement of certain species. Ecological 




SUMMER 1918 
WINTER 1916-18 




SUMMER 1921 
WINTER 1919-21 




SUMMER 1926 
WINTER 1924-26 




SUMMER 1932 
WINTER 1930-32 




FIG. 12.1. Westward dispersal of the starling as shown by advancing limit of 
breeding range (solid line) and winter records (black dots) outside the breeding 
range. Star indicates unusually advanced breeding record in 1934. (Kcssel, 

1953.) 

barriers may be either physical or biological. The action of many 
physical barriers is easily understood since they may be too wide to 
cross all at once and unsuitable as a way station because of local con- 
ditions. A great expanse of salt water or of dry land acts as an effec- 
tive barrier against the dispersal of fresh-water forms. A rugged 



Ecesis 429 

mountain range is a barrier preventing the dispersal of plants that 
require a warm, moist soil for their growth. 

Biological barriers are sometimes more obscure in their mode of 
operation, but they may be equally effective in preventing the spread 
of certain species. The action of biological barriers is perhaps most 
easily seen when larger plants are involved. If an extensive area is 
completely occupied by trees, for example, other species of trees with 
closely similar ecological requirements cannot enter the area. If the 
forested area is large, the dispersal of the other species will be stopped 
unless their seeds can be carried over or around the barrier. In the 
same way a grass sod acts as a biological barrier to the dispersal of 
other herbs from adjacent areas. Undisturbed prairie grass regularly 
prevents the invasion of trees even though in some situations the 
climate can "support" trees once the prairie sod is broken. Com- 
munities in which all available space for certain types of organisms 
has been occupied are termed closed communities as far as these kinds 
of organisms are concerned. Although these communities thus pre- 
vent the invasion and the dispersal of certain species, they may allow 
species of very different requirements to enter. 

Ecesis. In order for new species to invade an area they not only 
must have some means of reaching the new locality but also must be 
able to grow and reproduce under the conditions found there. Or- 
dinarily the first invaders of a bare area are plants, and for them the 
physical features of the soil and the climate are of primary importance 
in determining whether or not ecesis, or successful establishment, can 
take place. The pioneer assemblage usually of plants first estab- 
lishing itself in a new area is sometimes referred to as a colony but 
care should be taken to avoid confusion with the use of the same 
term for groups of social animals. After the ecesis of the pioneers 
other species will arrive at the area and will gain a foothold if they 
can. The presence of certain plants is usually a primary factor in 
determining whether or not invading animal species will be able to 
establish themselves. Accordingly, the first animals to arrive will be 
primarily concerned with whether the vegetation which they find is 
suitable. The general dependence of the distribution of animals 
upon that of plants is not limited to the early stages of colonization, 
but applies in more advanced communities, especially in the ter- 
restrial environment. 

Later arrivals at the area being invaded must be able not only to 
tolerate the physical conditions but also to compete successfully with 
the species already present if they are to establish themselves. The 
absence of a species in a given area does not necessarily mean that it 



430 Succession and Fluctuation 

could not live there as far as the climatic and edaphic conditions are 
concerned. Finally the newly established species will be forced to 
defend themselves against additional invaders that continue to arrive. 
The success with which a species can extend into a variety of new 
regions depends on its ability to tolerate widely different ecological 
influences both physical and biological. 

After the invading species becomes established in a new biotope, 
it may undergo genetic changes. This will further complicate or 
modify the situation and the effects of such alterations in the inherent 
nature of the members of the community must be taken into account, 
especially in any study of an area extending over many generations of 
the inhabitants. The consequences of mutation and hybridization 
and the effects of isolation are topics beyond our present scope and 
the reader is referred to specific treatments such as those of Mayr 
(1942), Cain (1944), or Anderson (1949). 

In summary we observe that from the point of view of the species 
a tendency for dispersal is always present, When pioneers reach a 
new area, a struggle for establishment takes place, and, if the struggle 
is successful, genetic modification or evolutionary change may ensue. 
Looking at the same process from the point of view of a specific area 
we may observe the change of inhabitants as time passes. If we 
watched one spot for a long period of time, we would witness a suc- 
cession of communities each one formed from new arrivals, allowed 
to flourish for a while, and then replaced by a new community. 

Succession and Climax 

The species that have successfully invaded a biotope dominate the 
scene for a period and form a closed community; further arrivals 
cannot at first establish themselves. However, in the course of time 
conditions become altered with the result that the members of the 
existing community no longer compete successfully with the invaders. 
A new dominant type gains a foothold, and a new community suc- 
ceeds the old. By the modification of the environment one com- 
munity puts itself at a disadvantage and gives way to another; com- 
munities appearing at later stages of ecological succession are estab- 
lished partly or chiefly because of the modifying action of earlier 
communities. 

One community continues to follow another until in many situa- 
tions a type of community is reached that cannot be displaced under 
the prevailing conditions. The community that can maintain itself 



Succession and Climax 431 

indefinitely in each biotope is known as the climax. In other situa- 
tions doubt exists as to whether the communities would ever be un- 
disturbed for long enough to enable an equilibrium condition or 
climax to be reached. For example, waterways in the black spruce 
muskeg of Alaska cause melting of the frozen ground with a conse- 
quent destruction of the spruce vegetation through the caving in of 
the surrounding terrain and a conversion of the area to treeless bogs. 
In time the spruce forest with frozen ground becomes reestablished 
and the cycle appears to continue indefinitely (Drury, 1952). In 
such situations the whole area might be considered as being in the 
climax condition and the alternation of vegetation types merely fluc- 
tuations within the climax. Possibly analogous fluctuations or cyclic 
changes in the fauna and flora occur in the oceanic community. This 
type of fluctuation within communities that are apparently in the 
climax condition will be further discussed in the latter part of this 
chapter. 

When a climax community has become established, it tends to 
remain in possession of the area because it does not change the en- 
vironment so as to injure itself or to favor the growth of different 
dominant species, and because its members can resist all competition 
from the outside. The succession of communities leading to a recog- 
nized type of climax is termed a sere. Seres composed of different 
sequences of communities typify different situations. 

On land the type of climax community in which the sere culminates 
is often determined primarily by the climate, and similar climax com- 
munities dominated by plants often extend over large regions. In 
local areas with special edaphic or physiographic conditions, or sub- 
ject to recurring fire or disease, a type of community different from 
the surrounding region may be displayed. Clements (1936) and his 
followers originally believed that given sufficient time every local area 
would eventually develop the same type of climax community the 
type characteristic of the region as a whole under the prevailing 
climate, and that the continuing existence of a different type of com- 
munity in a local area was due to "arrested succession." Subse- 
quently ecologists have abandoned the strict application of the mono- 
climax theory based on climate alone, and many have adopted a 
polyclimax theory in which the type of community that maintains 
itself in each area is regarded as the climax for that area. Since 
communities do not form a sharply delineated mosaic but grade into 
one another with many variations of species composition according to 
local conditions, Whittaker (1953) has proposed a climax pattern 



432 Succession and Fluctuation 

hypothesis in which "vegetation is conceived as a pattern of popula- 
tions, variously related to one another, corresponding to the pattern 
of environmental gradients." 

The climax community remains in possession of the area until some 
unusual change causes its displacement. The biotope and its biota 
may be completely destroyed by a cataclysm such as a volcanic erup- 
tion or extensive erosion; serious but incomplete destruction may 
result from a forest fire, flood, or hurricane. On the other hand, the 
climax community may be displaced less violently by removal of the 
dominant species through lumbering or the attacks of parasites. 

Types of Succession 

Primary Succession. Ecological succession that begins on a bare 
area where no life has existed, or where the previous fauna and flora 
have been completely destroyed, is known as primary succession. 
Habitats that become available for initial colonization include: new 
islands, sand bars, deltas, or glacial moraines; recently formed ponds; 
fresh alluvial, shore, or volcanic deposits; and various types of sub- 
strata exposed by erosion. These diverse areas may be classified as 
xeric, mesic, or hydric according to whether the initial moisture con- 
ditions are dry, intermediate, or wet. Seres starting from these types 
of situations represent xerarch, mesarch, and hydrarch succession, 
respectively. 

A striking example of primary succession, and a classical one, is 
the hydrarch succession in which a pond and its community is con- 
verted to dry land with an entirely different community. The vege- 
tation rooted along the margins of the pond is able to push out from 
shallow water into deeper water in a variety of ways ( Fig. 12.2 ) . As 
the vegetation invades the open water, the margin of the pond is 
reduced. At the same time the growth of the plankton and of other 
aquatic organisms adds organic matter, and much of this is deposited 
on the bottom. Beavers, muskrats, and other animals carry material 
into the pond, deciduous vegetation blows in from the shore, and silt 
is carried in from surrounding land. Rafts of vegetation from the 
pond margin drift offshore, strand, and take root, thus establishing 
islets that grow in size until they meet and also join the shore. At 
the same time the area available for completely aquatic plants, such 
as the water lilies, becomes reduced. As the free water is changed 
to swampy land, the water lilies and similar species give way to 
sedges and rushes, and these are subsequently replaced by heaths and 
shrubs. As succession continues, the soil is further built up, so that 





it becomes drier and is also changed 
chemically. In time certain smaller spe- 
cies of trees invade the area, taking the 
place of the shrubs, and eventually full- 
sized forest trees will dominate the scene. 
Various stages in such a succession are 
shown in Fig. 12.3. 

The existence of zonation in a com- 
munity does not necessarily mean that 
succession is going on since distinct hori- 
zontal subdivisions may occur in a static 
community, as described in the previous 
chapter. Furthermore, the change in 
the vegetation is not always self -induced 
but may be caused by outside influences. 
Sometimes the conversion of a swamp to 
dry land is brought about primarily by a 
lowering of the water table caused by a 
physiographic change. But in other in- 
stances, as in the situation described in 
the preceding paragraph, the vegetation 
itself is chiefly responsible for building 
up the land as ecological succession goes 
forward. 

At the same time that the vegetation 
is undergoing these profound changes in 
the hydrarch succession, the animal life 
of the community is correspondingly 
altered. Fish, beavers, and muskrats 
will gradually be excluded and terrestrial 
vertebrates will enter. Less conspicuous 
but just as significant will be the mani- 
festations of succession among the in- 
vertebrates and the microorganisms. 
These trends in the animal members of 
the community are indicated schemati- 
cally in Fig. 12.4 for a hydrosere in Illi- 
nois. The changes in bird species asso- 
ciated with a hydrarch succession are 
shown in Table 19. The changes in the 

FIG. 12.2. Five methods by which vegetation invades deeper water from the 
pond margin: (a) spike rush, (b) tussock sedge, (c) loosestrife, (d) cat-tail flag, 
and (e) sphagnum and heath. (Needham & Lloyd, 1937, Copyright, Cornell 

Univ. Press.) 
433 




d 




434 



Succession and Fluctuation 




FIG. 12.3. Hydrarch succession in Gifford Bog, Falmouth, Massachusetts. Pond 
lilies (Nymphaea)> bulrushes (Scirpus), and masses of filamentous algae fill in 
the open water. Cassandra or leather leaf (Chamaedaphne) forms islets and also 
invades the bog at its margin, where it is followed by sweet pepperbush (Clethra 
alnifolia) and later by forest vegetation. 



Primary Succession 



435 




Bare 
bottom 
(pioneer) stage 




Temporary 
and prairie 



POND SUCCESSION 



Beech and 
maple forest 
(climax) stage 



FIG. 12.4. Schematic representation of some of the changes in animal life during 
successional stages from pond to forest in Illinois. ( Buchsbaum, 1937. ) 

insect members of serai communities leading to a red oak-maple climax 
are discussed by Smith (1928). 

Secondary Succession. When a natural area is disturbed so as to 
destroy the community inhabiting it and to set back the course of 
succession, the new series of communities tending again toward the 
climax constitutes secondary succession. This situation arises when 
the principal species of the community have been destroyed by fire, 
disease, tornado, flood, or by human activities such as farming or 
lumbering. In some instances of secondary succession a community 
is established that is essentially the same as a stage in the previous 



436 



Succession and Fluctuation 



primary succession, but in other instances a quite different community 
is brought into being by the special conditions resulting from the dis- 
turbance. However, later stages tend toward the type of community 
found in the primary succession, and ordinarily the same climax is 
eventually reached. 

An admirable illustration of secondary succession may be observed 
today in old fields of central New England, and the phenomenon oc- 
curred on a large scale during the last century when wholesale farm 
abandonment took place. The early settlers of this region cut down 
the forests to build farms and homesteads. Where the land was in- 
tensively cultivated, the stumps and roots of the forest trees were 

TABLE 19 

REPRESENTATIVE SPECIES OF BIRDS PRESENT IN VARIOUS STAGES or A 

TYPICAL HYDROSERE IN NORTHEASTERN OHIO TO SHOW THE CHANGE IN 

BIRD LIFE THAT ACCOMPANIES THE CHANGE IN VEGETATION 

(Aldrich, 1945) 



\. Stage 


1 


2 


3 


4 


5 


Bird Species \. 


Water 
Lily 


Loose- 
strife- 
Cattail 


Button- 
bush- 
Alder 


Maple- 
Elm- 
Ash 


Beech- 
Maple 


Pied-billed grebe 


N 


S 








Common 1 mallard 


X 


8 








Virginia rail 




S 


X 






Long-billed marsh wren 




S 


X 






Eastern red-wing 




S 


8 






Eastern swamp sparrow 




S 


8 






Eastern kingbird 






S 






Alder flycatcher 






S 






Eastern yellow warbler 






S 






Catbird 






S 


X 




Eastern goldfinch 






S 


X 




Northern yellow-throat 






S 


S 




Mississippi song sparrow 






S 


S 




Northern blue jay 








p 




Eastern hairy woodpecker 








p 


P 


Northern downy woodpecker 








p 


P 


Eastern wood pewee 








S 


S 


Eastern white-breasted nuthatch 








p 


P 


Black-capped chickadee 








p 


P 


Tufted titmouse 








p 


P 


Red-eyed vireo 








S 


S 


Eastern oven-bird 








S 


S 



X = present at times; P * permanent resident; 5 seasonal. 



Secondary Succession 437 

completely removed from the soil, and the cleared areas were planted 
to crops or used as pastures. Beginning shortly after 1830 the open- 
ing of rich farmlands in the west, the building of railroads, and the 
growth of industrial centers, brought about the exodus of the farmer 
from the rocky hillsides of New England that were so difficult to work. 
The abandoned fields were covered by a thick sod of grass. In this 
turf the seeds of white pine trees blown from neighboring forested 
areas found a favorable environment for germination, but other forest 
trees were not able to establish themselves effectively in the aban- 
doned fields. The result was that the white pines grew into a dense, 
uniform stand, and eventually produced a forest in which all the 
dominant trees were of this species. A few hardwood saplings later 
became established as an understory throughout the pine forest, but 
their further growth was suppressed by the pines. 

In the course of fifty years or so, the pines had grown to a size that 
made a valuable timber harvest possible. Great areas were lumbered 
off; in regions where lumbering was not carried out the even-aged 
trees grew old and finally fell. In both situations the trees which 
then grew into dominant position were not white pines but were 
hardwood trees that had existed in subordinate position almost un- 
noticed in the pine forest. Pines will not sprout from stumps, and, 
being an intolerant species, the pine seedlings are unable to develop 
in the shade of the mature trees (Fig. 12.5). The root sprouts and 




FIG. 12.5. View of the forest floor in a stand of mature white pines in southern 
New Hampshire, showing the absence of pine seedlings but the presence of beech 

and maple saplings. 



438 Succession and Fluctuation 

seedlings of beeeh, maple, and other tolerant species that existed as 
minor members of the community grew into mature trees when they 
were "'released" by the removal of the pines. White pine stands 
formed in abandoned fields of New England were therefore not self- 
perpetuating. Once the mature pines were gone, the species com- 
position of the forests changed completely, and gave way to a com- 
munity of mixed hardwoods. Thus, secondary succession originating 
in old fields of this region produces a transient community of pines 
but eventually results in the restoration of the typical climax forest. 

Convergence. Since the bare areas in which primary succession 
can be initiated are extremely diverse, it is not surprising to find a 
correspondingly great variety in pioneer communities. As succession 
proceeds, however, the different biotopes tend to be modified toward 
a more nearly uniform condition that will support similar communi- 
ties. This convergence is particularly clear in relation to the moisture 
factor serai communities cause hydric habitats to become drier, and 
xeric habitats to become moister, so that a trend toward a more nearly 
mesic condition is followed from either extreme. As a general prin- 
cipal we recognize a convergence in succession such that seres orig- 
inating in diverse habitats within a region of similar climate tend to 
progress toward similar climax communities. 

An examination of succession in the deciduous forest region of 
Indiana revealed a tendency to converge that could be traced in seres 
originating from five kinds of biotopes as shown in the scheme on page 

439 from Clements and Shelf ord (1939). Pioneer communities 
established on a sand ridge, clay bank, flood plain, shallow pond, or 
deep pond initiated the successions indicated by the serial numbers 
with the dominant plants named. The exclusive presence, or relative 
abundance, of a dominant plant served as a serai index to the stage 
of succession in each sere. An animal species characteristic of each 
stage could also be recognized, and the names of these animal serai 
indices appear in the diagram. Succession in each of the widely 
different original areas tended to bring about the eventual establish- 
ment of the same type of climax community in which beech and sugar 
maple trees and the salamander, Plethodon cinereus, are the indices. 

Succession in Special Habitats. The instances of succession dis- 
cussed above are typical of broad areas in temperate regions. Other 
manifestations of succession occur in special habitatseither large or 
small. A classical example of succession in a microhabitat is the 
sequence of bacteria, protozoans, and other microorganisms that follow 
one another in a hay infusion formed by allowing hay to rot in a 
quantity of water. The seasonal succession of different kinds of 



M 

I 

*1 3 "H 

hJ p -S 



?s tn 

!^ ~& 

I 

en 



o 
o 

ll 

^> ^ 

S^ C 

"o o 



I 

I a 

f= c5 

M C* 

5 - 



53 ^_^ 

O ^ -? 
(^ Q,^ p < 

CO ffi 



Ib 

fc o 

815 
2 S 
a -i 

1 



1 
I 



8 









5 
fel 


catenalus 

-lily 

/iwm partumeium 
DEEP POND 








11 


03 Ui S 


OJ 

s 


inereus 


w a 

2 b. 


</a * 3 
^> -+J 

153 


131 
^^^ 

OQ f-H 






eg 









^ 


S-2 


^^^ 








eg ^ 






^ 


" 


H&; 


00 




o 


** 









S 
o 



CO 
fc 



w 

I 





0< r- 




440 Succession and Fluctuation 

phytoplankton and of zooplankton in natural waters is in part caused 
by temporary modification of the medium by the organisms them- 
selves and in part by seasonal changes in the physical environment, 
as discussed in Chapters 8 and 13. The special circumstances of suc- 
cession in arctic areas in which the substratum is repeatedly dis- 
turbed by frost action are described by Hopkins and Sigafoos (1951). 

Succession on a small scale, but of a complex nature, takes place 
on and in the trunks of fallen trees. The sound wood, cambium, and 
bark are first attacked by a group of boring insects, saprophytic fungi, 
and various kinds of microorganisms. This pioneer assemblage is 
followed by a series of more elaborate communities that cause and 
accompany the further disintegration and decomposition of the tree 
trunk. The presence of these organisms attracts predators and 
scavengers until a very large number of species may be represented 
in the dead-tree-trunk biotope. Mosses, lichens, ferns, and, later, 
higher plants find the rotting log a favorable substratum for growth. 
With the establishment of autotrophic plants a new cycle of con- 
structive growth begins, in which the new pioneers take root literally 
as well as figuratively from the remains of the previous cycle. 

The building up of coast lines is another activity in which the course 
of succession and the physiographic changes are mutually interde- 
pendent. The ecological steps involved in and following the forma- 
tion of sand dunes along the southern shore of Lake Michigan as one 
community succeeds another have been studied by a number of ecol- 
ogists. In simplest terms the course of succession was found to in- 
volve the capture of moving sand by grass, and the development of 
communities dominated successively by cottonwoods, pines, and oaks, 
leading to the beech-maple forest. Subsequent investigation has 
shown that the succession is far more elaborate and also more flexible 
than was at first supposed and that several interlocking channels for 
advancement or recession may be followed by the various series of 
communities involved, as indicated in Fig. 12.6. 

Along marine coast lines in tropical regions a well-known land- 
building succession takes place (Fig. 12.7). Here red mangroves 
( Rhizophora mangle ) work out from the shore by dropping viviparous 
seedlings that will root only in water more than about 25 cm deep. 
By means of spreading, stilt-like roots this species of mangrove can 
maintain itself in and slightly below the tidal zone in spite of wave 
action. Mud that collects around the dense jumble of roots builds up 
the bottom, causing the gradual elimination of the red mangrove, and 
prepares the way for the establishment of the black mangrove 
(Avicennia nitida), the seedlings of which will grow only in water 



Succession in Special Habitats 



441 









Hemlock 




E 


1 


E 


|I 


1 


E 






n 


a 


E 



_2 G 

rt .2 



S ~ >> 

.2 M 

w s 



Q 



g ^ 



: Q 
O) (^ Q. 



18 




442 



Succession and Fluctuation 



shallower than about 25 cm. As the bottom is raised and the soil 
becomes drier, buttonwoods replace the mangroves, and these are 
subsequently succeeded by the climax palm community. 

In the situations described thus far the controlling organisms are 
plants. On land, plants are usually the dominant members of the 
community, and vegetation development is chiefly responsible for 
causing succession, but in some communities animals are found to 
exert control. For example, birds nesting in colonies are sometimes 
so abundant that their activities and their droppings cause a change 
in the vegetation. In this way a rookery of herring gulls on Kent 
Island off the coast of New Brunswick caused the elimination of a 
grove of spruce trees and its replacement by grass. 



MANGROVE SUCCESSION 



..Climax 
community 



Transition 



Salt-marsh Pioneer community 



Red 
mangrove 




High tide 
j> Seedling Lowt.de 

I*/ " ' Crtll lino 



Soil line 



Underlying rock 

FIG. 12.7. Diagram of succession along the margin of tropical shores as seen in 
southern Florida. (Modified from Davis, 1940.) 

Control of the environment by animals is more commonly found 
in aquatic habitats where sessile forms are prominent, The photo- 
graphs in Fig. 12.8 show two stages in an all-animal warfare in the 
tidal zone. Oysters became established on the sea bottom in this 
biotope, but mussels soon began attaching to the oysters' shells. 
Gradually the growth of the mussels smothered the oysters, and the 
latter were replaced by an almost continuous carpet of mussels. Sub- 
sequently, barnacles became attached to the shells of the mussels in 
sufficient numbers to kill them. After the death of the mussels their 
shells broke loose from the bottom and the barnacle population was 
swept away by wave action so that no enduring change was brought 
about in this instance. 

Frequently both plants and animals are involved in well-defined 
succession in the littoral zone as described by Dexter ( 1947 ) for mud- 



Succession in Special Habitats 443 

bottom communities; similar relationships occur on hard bottoms and 
on submerged surfaces subject to attack by fouling organisms. When 
a jetty or pier is built, or a boat without anti-fouling paint is left at 
her moorings, various plants and animals attach in recognizable se- 
quences. Bacteria are the first to establish themselves, and they com- 
monly form a film in which benthic diatoms and various filamentous 
algae find a foothold. The subsequent course of succession varies 
according to circumstances, but in some situations Bryozoa appear 




From Miner, 1934 

FIG. 12.8a, Mud flat community dominated by oysters. The shells of these ani- 
mals provide a suitable substratum for the subsequent attachment of mussels. 




From Miner, 1934 

FIG. 12.8k. Mussels have overgrown the oysters and will in turn be smothered by 
the barnacles that are seen beginning to attach in abundance to the mussel shells. 



444 Succession and Fluctuation 

next, followed by mussels (Scheer, 1945), whereas in other situations 
barnacles, tube worms, or tunicates attach at an early stage (Woods 
Hole Oceanographic Institution, 1952). The varied surfaces of the 
attached organisms, and particularly the crevices between them, pro- 
vide abodes for a host of mobile forms that join the community in 
these later stages and that could not inhabit the biotope if it were not 
for the presence of the earlier arrivals. Thus, in these small habitats, 
as well as in the larger areas considered, the early inhabitants change 
the conditions so as to cause their own displacement and the estab- 
lishment of successive new communities. 



Modification of Succession 

The course of succession may be modified by unusual natural 
circumstances, or frequently by the hand of man. Succession may 
be changed, for example, by the browsing of an overly abundant 
population of deer in a forest. Deer do not browse on all species of 
the vegetation indiscriminately but have certain favorite items. If 
the deer become numerous, the most appetizing plants will be grazed 
down excessively and the unpalatable types will be given an ad- 
vantage. In this way the species composition of the vegetation and 
the course of succession may be considerably altered. If man permits 
overgrazing by cattle on the range, the usual sequence of succession 
may be reversed so that the vegetation is pushed back from perennial 
grasses to annual weeds (Graham, 1944, Ch. 9). 

Repeated forest fires sometimes bring succession to a halt at a com- 
munity stage that is different from the climax in neighboring areas. 
In the sandy soils near Plymouth, Massachusetts, and on Cape Cod 
the prevailing vegetation consists chiefly of pine and scrub oak, and 
this appears to be a climax condition determined partly at least by 
fire. On an undisturbed island in a large pond within this region a 
mature stand of beech, maple, oak, hemlock, and yellow birch has 
developed. This island has not been burned over during the memory 
of present residents, and it may well be that because of the protection 
of the water no fire has occurred on the island for generations. The 
existence of this island community suggests that other neighboring 
regions might progress beyond the pine and scrub oak stage if the 
interference of forest fires was prey0lited for a sufficient time. The 
pinelands in some parts of southeastern United States may similarly 
represent a climax community controlled by fire rather than by climate 
(Fig. 12.9). 

Modification of succession by special factors may also be observed 



Modification of Succession 445 

on a small scale in many local habitats. One example is furnished 
by the activity of certain animals in maintaining open "glades" on the 
surface of rocks in the tidal zone (Fig. 12.10). The browsing of 
limpets at the margins of their territories shears away the enlarging 
basal holdfasts of the reel algae that would normally grow over *he 
rock surface in the course of succession. Another animal, Idotea 
viridis, bores holes in the fronds of the algae so that they break away 
easily and thus assists in affecting succession in \his microhabitat. 

In contrast to the situations mentioned above the activities of man 
may sometimes ' add a stage beyond the usual climax. In certain 
parts of southern California the climax vegetation formerly consisted 
of grass or chaparral. When man introduced the eucalyptus from 
Australia, this tree established extensive stands replacing the former 
vegetation and adding a forest stage to the previously recognized limit 
to succession in the region. 

Ecologists have taken such an interest in pointing out the occur- 
rence of succession and its importance under various circumstances 
that emphasis has failed to be laid upon the fact that in some regions 




Photo by H. B. Moore 

FIG. 12.9. Pineland community in southern Florida, consisting principally of 

pines, palmettos, and sawgrass. After such an area is burned over, the same type 

of community is reestablished directly. 



446 



Succession and Fluctuation 



no succession appears to take place. Where conditions are not 
changed by the inhabitants of an area so as to favor other species, 
succession does not occur. If the existing community is destroyed in 
many regions of the tropics or subtropics, the area will be repopulated 
directly by the original species. The communities of the open ocean 
and of the deep sea may be regarded as being in a climax condition. 
Since the physical conditions of the sea and also of large lakes are 
essentially unmodifiable by the plant and animal inhabitants, no eco- 
logical succession takes place that is comparable to that found on land. 
In the open ocean seasonal and short-term sequences in the communi- 
ties of temperate regions occur, as they do on land, but growth of 
marine organisms in the plankton or on the floor of the deep sea does 
not change the nature of the physical environment in such a way as 
to cause a permanent or irreversible replacement of one community 
by another that regularly extends beyond the seasonal cycle. For a 
further discussion of ecological succession the reader is referred to 
Costing (1948), Allee et al. (1949, Ch. 29), and for special situa- 
tions to Hutchinson (1941), Dansereau (1951), and Niering (1953). 




Photo by C. Davidson 

FIG. 12.10. Open "glades" maintained by limpets (Patella) on rocks in the tidal 

zone of the Isle of Cumbrae, Scotland. The browsing of these gastropods (seen 

on the large rock in the foreground ) curtails the invasion of the red alga, Gigartina 

stellata. Barnacles also inhabit the "glades." 



Community Type 447 



COMMUNITY CLASSIFICATION 

Since the community is essentially a functional unit, the size of the 
area occupied by the community, the number of living things included 
in it, and its organization are variable according to circumstances As 
we have seen, the groups of plants and animals that constitute a com- 
munity may be large or small, and frequently one functional group is 
contained within another. Furthermore, one community re^aces 
another in the same area as ecological succession proceeds. This 
situation presents difficulties when we attempt to arrange communi- 
ties in some sort of order since any practical classification is based 
on descriptive considerations such as the life form of the dominant 
species or their position in the terrain. The presence of some types 
of communities is more strongly influenced by climatic factors than 
edaphic factors, but for other types the reverse is true. 

Studies of the arrangement of communities in hierarchies have 
been made chiefly in environments where plants hold the dominant 
position in the biocenose. It is not surprising then that common sys- 
tems of classifications are based primarily on the vegetation./ We shall 
describe the chief units into which communities are grouped, but 
unfortunately complete agreement does not exist as to terminology. 

Community Type 

Communities of a certain dominant life form, such as deciduous 
forests, coniferous forests, and the like, obviously compose recogniz- 
able categories. Some authorities, like Tansley (1939), regard any 
mature community of distinctive life form as a "formation," but other 
authorities, following Clements (1936), apply this term to climax 
communities only. Extreme difficulty is often experienced in ascer- 
taining whether the vegetation in a given region has attained its 
climax condition, and for many types of investigation it is not neces- 
sary to do so. Also there seems little logic in using a different term 
for, say, grassland that is climax and grassland that is on its way to 
becoming forest. When the state of ecological succession is known, 
a modifier may be used for clarity by referring to the community as 
a climax formation or a serai formation. Until general agreement on 
usage is reached, it will be necessary to specify the particular sense 
in which the term formation is used on each occasion, or to avoid it 
by referring to the life form of the dominant organisms or to the 
community type. Thus we may state that the vegetation in a certain 



448 Succession and Fluctuation 

area is a serai shrub formation, is of the shrub life form, or is of the 
shrub community type. 



The Biome 

Certain communities whose dominant species have a distinctive life 
form have become more or less permanently established in certain 
climatic regions of the earth and are believed to be in the climax 
condition. Associated with these climax communities are communi- 
ties in earlier stages of ecological succession and also communities of 
a type controlled by special local conditions different from the general 
nature of the region. Such a complex of communities, characterized 
by a distinctive type of climax community and maintained under the 
climatic conditions of the region is known as a biome. All animal 
and plant components of each of the included communities are mem- 
bers of the biome. The biomes constitute the great regions of the 
world distinguished on an ecological basis, such as the tundra, the 
desert, the grasslands, and the various forests, > 

The biome consists of a special combination or complex of com- 
munities. The essential matrix of the biome is composed of climax 
communities with dominants of a certain life form that give the biome 
its particular character. Communities of different life form are pres- 
ent as minor constituents of the biome. The major climax communi- 
ties in the biome are of the same type but differ in species composition 
in different parts of the biom^ The deciduous forest biome that 
extends across the eastern part of the United States, for example, is 
characterized by a community type in which deciduous trees are 
dominant.- Major communities of equal rank in this deciduous forest 
biome are the oak-hickory forest of the central Atlantic states, the 
beech-maple forest of the Middle West, the hardwood forest of 
northern New England, and others of the same life form. These 
major climax communities that form the essential matrix of the biome 
have been termed associations by Clements and his followers. The 
life form characteristic of the major climax communities and hence 
the general character of the biome is determined primarily by the 
nature of the regional climate. Which of several major communities 
of the same type will be present in each part of the biome is de- 
termined by local variations in both climatic and edaphic conditions. 

Within each community Clementsian ecologists recognize certain 
subdivisions. Geographical differences in abundance or in relation- 
ship of the dominants in the community are called faciations. Within 
a region occupied by a grass community, for example, variations in 



The Biome 449 

climate with latitude and with altitude result in differences in the 
selection of dominant species. Each of these various geographical 
combinations of dominants constitutes a faciation of the community. 
Variations of the dominant species on a more local scale when deline- 
able are termed lociations. 

The subdominant members of a community may also form recog- 
nizable groups. These subdivisions have been called "societies'" but 
this term is unsatisfactory in this connection because of its specific 
use for highly integrated animal groups within the same species as 
seen in insect colonies. If we again take a climax deciduous forest 
as an illustration, the shrubs, seasonal herbs, and cryptogamic plants 
as well as the various categories of animal life may be regarded as 
constituting definite groups of subdominants. Many aquatic com- 
munities including benthic plant and animal groups may be similarly 
subdivided, but with the smaller life forms local subdivisions are not 
as conspicuous as in communities dominated by the larger vegetation. 
Subdivision of communities involving plankton and nekton are even 
less clearly defined because of the mobility of the medium and the 
organisms. 

Many ecologists feel that communities and their subdivisions cannot 
be as clearly distinguished as suggested by the foregoing terms, and 
that no system of community classification is really satisfactory. 
Some investigators feel that each community is a law unto itself, as 
argued by'Gleason (1926), since each is composed of whatever plants 
and animals have reached the area and have found conditions toler- 
able. ; As the inhabitants in each situation have grown and multi- 
plied, the community and its environment have changed until a stable 
condition has been reached. If similar climax communities develop, 
it is because similar conditions happened to exist, but an indefinite 
amount of individual variation is possible. 

Regardless of the method of classifying communities, each biome 
is found to consist of several major communities (or associations) in 
the climax condition and of many minor communities.. Between or 
within the major climax communities the developmental stages of 
these communities can be recognized and also other community types 
produced by local variations in the environment. In the deciduous 
forest biome, for example, grass and brush communities are found that 
may eventually mature into forest communities. Also present are the 
communities of ponds, marshes, rock ledges, and sand hills that will 
not be converted into deciduous forest for a long time but are definite 
members of the biome.j A certain amount of cohesion within the 
biome is provided by plant species common to two or more communi- 



450 



Succession and Fluctuation 



1H TUNDRA AND ICE 
TAIGA 

TEMPERATE DECIDUOUS FOREST 
GRASSLAND 

mm DESERT 

TROPICAL FOREST 
TEMPERATE RAINFOREST 




FIG, 12.11, Generalized representation of the major biomes of the continents. 
(Modified from Goode, 1943, Copyright, Rand McNally and Co.) 



Fluctuation within the Community 451 

ties and by the mammals, birds, and other animals that move back 
and forth between various communities of the biome. For example, 
beavers are members of a pond community within a forest biome and 
ako are members of an aspen community that borders the pond. 
LThe biomes are the result of the equilibria established by living 
inhabitants with all aspects of the climate of the region. Since the 
biomes are characterized by major communities of distinctive life 
form, they correspond to certain of the life zones that are based on 
the same life forms. The chief biomes that occur in a series frp) the 
equator toward the poles coincide with the principal continental life 
zones described in Chapter 5. (jjjome biomes tend to be continuous in 
extent and form a more or less definite unit, but others are discon- 
tinuous and the parts may be widely separated geographically. 
Communities of the same life form as a biome but too small to con- 
stitute a separate biome are referred to as biome types. 

The clearly defined major biomes of the land masses of the world 
are shown in Fig. 12.11. The sea may be regarded as constituting 
an additional biome, but the plants of the open ocean do not exert 
the controlling influence on the biotope experienced on landi/ All 
parts of the sea are interconnected, and many kinds of plankton and 
nekton move readily from one region to another. The biogeographic 
subdivisions of the sea are therefore based principally on physical 
features of the environment, as has been described in previous chap- 
ters and is further elaborated by Ekman ( 1953 ) . 

The principal biomes on land are: tundra, taiga, temperate decidu- 
ous forest, temperate rain forest, grassland, desert, and tropical forest. 
In certain regions, as in central North America, the dividing line be- 
tween biomes tends to run longitudinally, but for the most part the 
chief biomes of the world are arranged in a general latitudinal se- 
quence. For a fuller description of the biomes of the world the 
reader is referred to the general account of Cain (1944) and Allee 
et al. (1949, Ch. 30) and to suck special "treatments as Richards 
(1952) and Beard^(1953)... 

FLUCTUATION WITHIN THE COMMUNITY 

When the climax community has been established, can we then 
take a long breath and say that at last the ecosystem will be entirely 
constant? The answer is, No. Even within the relatively permanent 
and stable climax community fluctuations occur sometimes of con- 
siderable magnitude. It is true that in some situations the community 
remains relatively steady. Sometimes, the life cycles of the members 



452 Succession and Fluctuation 

of the population are staggered so that little change is apparent for 
the community as a whole. For example, in certain tropical regions 
where individual trees shed their leaves at different times, the vegeta- 
tion as a whole remains green throughout the year. Similarly, in 
tropical waters individual species of plankton in small numbers wax 
and wane, but the complexion of the community may continue essen- 
tially the same for long periods. At higher latitudes where climatic 
conditions change markedly during the year, the abundance of popu- 
lations and the composition of the community vary widely with the 
season. Yet even here the community may in some instances come 
around each season to essentially the same composition as in the 
previous year, resulting in a rather faithful repetition of the seasonal 
cycle. 

On the other hand, constancy within a community is the exception 
rather than the rule, even within the same broad climax condition. 
Every species in the community fluctuates to some extent in respect 
to rate of vegetative growth or of reproduction, and sometimes the 
variations are very large. In earlier chapters we have mentioned the 
various types of fluctuation of populations of individual species and 
also the reciprocal oscillations of certain populations of species ex- 
ploiting one another. We shall now consider fluctuations in com- 
ponent species against the background of the community as a whole. 
All but the most minor changes in the growth or reproduction of one 
species of a community is bound to have repercussions among other 
members of the biocenose. Sometimes the fluctuations are obvious 
and spectacular, as in a plague of mice or of locusts; on other occa- 
sions changes in the community originate in fluctuations among the 
unseen microorganisms of the soil or of the water, or among patho- 
genic bacteria or viruses. In either case the effects on the natural 
community may be serious, and they often have a drastic direct or 
indirect consequence for man. 

It is exceedingly difficult to ascertain the magnitude of fluctuations 
in the community in any precise terms. Attempts to take a census of 
wild populations have been made for a relatively small number of 
biotopes. Even in those instances in which a systematic count has 
been undertaken, the reliability of the census methods is hard to 
evaluate, and rarely have population measurements been continued 
sufficiently long to reveal significant relationships. The best available 
census data for extended periods are for species that are of concern 
to man either because they cause damage or because they provide a 
useful product. Records of abundance in wild populations have 
been kept for various disease organisms and insect pests, and for 



Irruptive Fluctuation 



453 



certain plants and animals harvested by man. Among the latter the 
most extensive records are those of trading posts for fur-bearing ani- 
mals and those of fishery agencies for the landings of commercial fish. 
We shall discuss some ecological aspects first of irruptive fluctuations 
in which a population undergoes wide, irregular swings in abundance, 
and then of oscillations that appear to be cyclic in nature. 

Irruptive Fluctuation 

Data from the commercial marine fisheries will serve admirably to 
illustrate the circumstances of irregular fluctuation in a community. 
It will be agreed that the general nature of the ocean has not changed 
drastically during the last several hundred years or so; yet during that 
time certain fish populations are known to have fluctuated in an ex- 
treme manner. In the cod fishery off northern Norway, for example, 
periods of scarcity severe enough to be recorded have occurred as far 
back as the time of Leif Ericson. In certain intervening years, as in 
the winters of 1714 and 1715, whole villages along the Norwegian 
coast are reported to have starved because of the failure of the cod 
fishery, whereas in the years immediately before and afterward codfish 
were plentiful. In the same region similar fluctuations have con- 
tinued to the present day, and equally extreme variations in the fish- 
eries occurred on the American side of the Atlantic. The fluctuation 
in the catch of mackeral by the American fishing fleet for a period of 
150 years shown in Fig. 12.12 may be taken as representative of the 
change in abundance of the mackerel population in the waters fished. 
The most spectacular change during this period occurred between 




FIG. 12.12. Fluctuations in the landings of mackerel (Scomber scombrus) by the 
fishing fleet from the east coast of North America. (From data of the U. S. Fish 

& Wildlife Service. ) 



454 Succession and Fluctuation 

1884 and 1886 when the catch of mackerel dropped from more than 60 
million kg to about 10 million kg. Subsequently the catch fell to a 
still lower figure but eventually rose again to a value of more than 30 
million kg. The seriousness of the ecological, economic, and socio- 
logical repercussions of these tremendous oscillations can easily be 
imagined. 

Year-Class Analysis. The economic importance of the fluctuations 
in the Norwegian herring fishery led the great marine biologist Johan 
Hjort to examine the annual differences in the population of the 
herring. By adopting an analytical approach to the problem he was 
able to trace the source of the oscillations of the entire catch to the 
special success of fish hatched in certain years. Hjort began by con- 
sidering a hypothetical, ideal situation in which the same number of 
young herring were spawned each year and in which mortality 
caused a uniform reduction in the abundance of each "year-class." 
Under these circumstances 1-year-old fish would always be the most 
abundant, 2-year-old fish the next most abundant, and so on. Since 
herring of 4 years of age and older are taken in the commercial catch, 
an analysis of the catch under these ideal conditions would show that 
the 4-year-old fish were always the most numerous, the 5-year-old 
fish were the next most abundant, and so on. 

Applying this reasoning to the catch in 1910, for example, the fish 
hatched in 1906 should be the most numerous, followed by the 1905 
year-class, and then by earlier year-classes, provided always that the 
same number were spawned each year and underwent the same mor- 
tality. When the age composition of the actual catch in 1910 was 
examined, however, a very different picture was presented, as shown 
in Fig. 12.13. The 1906 year-class, which was 4 years old in 1910, 
was by no means the most abundant, but occurred in extremely small 
numbers, and the 1904 year-class was found to represent more than 
80 per cent of the catch. Looking back in the diagram we see that the 
1904 year-class dominated the catch in 1909 and in 1908; this same 
year-class continued to dominate the catch for a good many years after 
1910 and remained recognizable until 1921. No other dominating 
year-class appeared until 1917 when the 1913 year-class entered the 
scene, but in 1922 the 1918 year-class became prominent. We are 
confronted then with the surprising discovery that during 20 years 
only three year-classes were successful. A good year-class is obviously 
the exception; in spite of the tremendous fecundity of the fish en- 
vironmental resistance is overpowering in most years. The fluctua- 
tion in the herring fishery, and in many others, is now known to be 



Year-Class Analysis 



455 



Age in years 

4 5 6 7 8 9 10111213 14 15 16 17 18 19 20 21 



iii 


i i i i i i i i i i i i I 
1907 


'*20- 


! 


1908 


20- 


i. 


1909 


40- 
20- 


j 


1910 


60- 
40- 
20- 


_ . 


1 1911 


60- 
40- 
20- 




11912 
_ 


60- 
40- 
20- 




11913 



60- 
40- 
20- 




..1 mi 


40- 
20- 


i. 


L _ i915 


40- 
20- 




11916 
. 


20- 


_ _ 1 1917 20- 


1. 


. 1 1918 


20- 


__^_J 


1 1919 


20- 




1 I 192 


20- 


J^ 


| 1921 


20- 


ill 


_ 1922 


20- 


.1. 


1923 




40- 
20- 


..1 


1924 


40- 
20- 




1 1925 


40- 
20- 


-r T 


- 1 . 1926 


%20- 


I T T T 
3456 


TTTTTTTTTTTTTi i 
789 101112131415161718192021 



FIG. 12,13. Age composition of catch of Norwegian herring (Clupea harengus) in 

each of the indicated years, showing the predominance of a few good year-classes. 

( Russell and Yonge, 1928, Copyright, Frederick Warne after Lea. ) 



456 



Succession and Fluctuation 



due, in part at least, to the occasional production of a year-class in 
which many more fish reach commercial size than in other years. 

The cause of the varying success of different year-classes is difficult 
to ascertain. Usually plenty of eggs are spawned every year; rarely 
does unusual mortality occur after commercial size has been reached. 
The critical point comes somewhere in very early life. Since the gen- 
eral nature of the ocean does not change, some subtle variation in the 
environment must arise during a sensitive stage of development. Per- 
haps an abnormal temperature in the spawning area is to be blamed; 
possibly a serious change in the abundance of planktonic members of 
the community-cither enemies or food organisms for the young fish- 
is responsible. Quantitative measurements of plankton in various 




Feb. Mar. Apr. May June July 
FIG. 12.14. Annual variation in average volume per standard net haul of a com- 
mon species of planktonic copepod in the waters between Cape Cod and Cape 
Hatteras. (Sears and Clarke, 1940, Biological Bull.) 



Cyclic Fluctuation 457 

parts of the ocean have revealed great variations in abundance from 
year to year. As an example, the drastic differences in the seasonal 
growth of a population of a planktonic copepod during 1929 to 1932 
are shown in Fig. 12.14. Presumably these great annual changes in 
the copepods influenced other members of the oceanic community, 
but no adequate explanation for these fluctuations in the plankton is 
known. Possibly the survival of young fish is dependent upon a 
simultaneous reduction in number of predators and an increase in 
availability of food. The relation between conditions favoring plank- 
ton growth and the abundance of young fish in the English Channel 
is discussed by Harvey (1950). Search for the factor or factors that 
control year-class success in specific instances is one of the many fasci- 
nating problems in ecology awaiting solution and obviously one of 
great practical significance. 

Cyclic Fluctuation 

The fluctuations of the population in some communities appear to 
be regular or cyclic. The phenomenon is particularly noticeable in 
high latitudes of the northern hemisphere among small mammals, cer- 
tain predators, gallinaceous birds, and some fish such as the salmon. 
The abundance of certain insect species and the rate of tree growth 
also exhibit cycles in various parts of the world. In addition, the 
occurrence of disease epidemics and other biological events are be- 
lieved by some to be definitely cyclic. 

Periodicity among northern mammals may be studied from the 
records of the Hudson Bay Company and other fur-trading agencies. 
As with commercial fisheries we must realize that the variations in the 
take by man may not represent the actual fluctuations of the entire 
population. Nevertheless the mass of evidence from fur traders is 
corroborated by other observations and leaves no doubt that oscil- 
lations of approximately uniform periods take place over long periods 
of time (Elton, 1942). Cycles of abundance with 3 to 4 years be- 
tween peaks are exhibited by mice, voles, and lemmings, and also by 
arctic foxes, martens, and snowy owls. Many of these cycles can be 
traced back in the records for a hundred years or more. The snow- 
shoe rabbit, the grouse, and certain insects are commonly reported to 
fluctuate with a 9- to 10-year cycle. Data for the Canadian lynx are 
available from 1735, but difference of opinion exists as to the size of 
the peaks that constitute maxima. If only the "big" peaks are counted, 
the average period of fluctuation is 9.6 years. If every year in which 
the population is greater than the preceding and the following year 



458 Succession and Fluctuation 

is considered to be a maximum, then analysis of the same records for 
the Canadian lynx yield an average of 5.8 years for the period of the 
cycle (Cole, 1951). Lynx populations in Finland oscillate with an 
average of 3.5 years for the cycle. The 10-year cycle may be inter- 
preted as the average distance between "high peaks" of a basic cycle 
with maxima 3, 3, and 4 years apart, of which the maximum after the 
4-year interval is larger than the others (Siivanen, 1948). 

Possible cycles of other periods have been reported and are sum- 
marized by Hutchinson and Deevey ( 1949 ) . The long-haired rat in 
western Queensland, Australia, appears to display an 11-year cycle. 
Irruptions of the chinch bug (Blissus leucopterus) between 1823 and 
1940 occurred with an average period of 9.6 years for the cycle, but 
individual periods varied from 7 to 13 years and the outbreaks are not 
considered to be truly cyclic. The population of guano-producing sea 
birds off Peru undergo catastrophic reductions about every 7 years 
when the warm current "El Nino" moves farther south than usual, as 
described in Chapter 5. 

Of a different nature is the periodicity of certain other insects in 
which the cycles are controlled by the simultaneous emergence of 
the adults after long, but often regular, periods of dormancy. Per- 
haps the most spectacular is the 17-year cicada (or "locust") Magi- 
cicada septendecim which in "locust years" swarms over the country- 
side but during the intervening years remains unseen as a nymphal 
stage in the ground. During one irruption in an oak-maple forest near 
Chicago these cicadas occurred at a density of 500,000 per hectare (50 
per sq m), equivalent to 77 kg of dry tissue per hectare (Strandine, 
1940). 

Fluctuation in growth of certain trees as revealed by the width of 
their growth rings often appears to be definitely periodic. The records 
studied by Douglas (1936) revealed a recognizable cycle of 9% to 10 
years. The growth of hemlock trees in Pennsylvania, traced for over 
230 years and of Douglas firs in Utah traced for about 1500 years 
oscillated with average cycles of almost exactly 3 years (Schulman, 
1948). Nevertheless, during the period 1703 to 1939 when the data 
for both tree species can be compared, the fluctuations in the two 
series appear to be completely independent. 

Causes of Fluctuation 

Discovery of the causes of fluctuations in communities is one of the 
most desirable objectives of ecological research but also one of the 
most difficult. We have seen that changes in the size of population 



Causes of Fluctuation 459 

are dependent upon the interplay of biotic potential and environmental 
resistance. Ordinarily the full biotic potential of animals and plants 
in nature is not reached environmental factors usually keep a strong 
check on the population. In some instances, however, even a slight 
variation in environmental resistance may produce a marked effect on 
abundance. Consider the accompanying hypothetical example for 
the annual recruitment of a fish population. From the point of view 

Number of fish spawned each year: 1,000,000 
1st year: 999,000 die; mortality 99.9%; 1000 left to catch 
2nd year: 998,000 die; mortality 99.8%; 2000 left to catch 
= 100% increase in number surviving 

of the original number of fish spawned the variation in mortality ap- 
pears to be trivial, but from the point of view of the survival of fish 
to form the adult population a very great difference is produced in 
the two sample years. 

In addition to variations in physical features of the environment, 
changes in the living elements, including both food supply and ene- 
mies, will produce modifications of the environmental resistance. The 
biotic potential of the prey is usually greater than that of the predator, 
but the predator can destroy a much greater supply of its prey than it 
actually assimilates. As a result predatory animals may sometimes 
be the chief factor holding prey species in check. When an unusually 
favorable combination of both physical and biological features of the 
environment occurs, the species "breaks out" from its controls. If the 
species possesses a high biotic potential, only a short period of un- 
checked reproduction will cause the population to irrupt. One North 
American species of field mouse can breed when it is 3 weeks old and 
can produce 13 litters per year. A little arithmetic will indicate the 
size to which the population could expand if most of the young sur- 
vived for a year or two. Many invertebrates, including notably the 
insects, can multiply even more explosively. After a population that 
has escaped from its usual control has increased for a period, its ex- 
pansion may eventually be stopped by another set of predators, by 
disease, or finally by a lack of food. The maxima of snowshoe rabbits 
are brought to an end reputedly by a disease that causes death from 
shock, possibly induced by shortage of some mineral element in the 
diet. 

Precisely which influence or combination of influences in the en- 
vironment is responsible for the augmentation of a population and 
which brings the increase of a population to a halt or causes it to re- 
cede is extremely difficult to determine. In attempting to solve the 



460 Succession and Fluctuation 

problem as complete a knowledge as possible must be obtained of 
the interrelations of the species in question with all aspects of its en- 
vironment at every stage in its life cycle. Apparently correlated con- 
ditions must be examined over a period of time and in different parts 
of the range. When it is feasible, the factor suspected of causing the 
fluctuation should be tested by laboratory or field experiments. 

Many illustrations of the control of growth and reproduction by 
individual environmental factors, or combination of factors, have been 
given in previous chapters, and others may be found in reports dealing 
primarily with this topic, such as those of Shelford (1951, 1951a). 
Following extensive investigations of the fluctuations in certain forest 
and non-forest animal populations in the Middle West, Shelford 
emphasized the fact that environmental influences exert their con- 
trol chiefly during sensitive periods in the life histories of the 
species concerned. Forest invertebrates, for example, were found to 
develop larger populations in years when rainfall is great during late 
March, April, and May with short dry periods, whereas for certain 
birds and mammals the conditions in February and March are more 
significant. 

Origin of Cycles. Because of the lack of sufficiently precise data 
we cannot at present state that the fluctuation of any population is 
strictly cyclic in the sense that accurate prediction of the times of 
maxima and minima can be made well into the future. Further study 
of the possible existence of truly cyclic behavior is very much desired 
since, in addition to the theoretical interest, prediction would permit 
precautions to be taken against outbreaks of pests and preparations to 
be made for glut or scarcity of fish, game, or furs. Since fluctuation 
may be only approximately cyclic, we must include random variations 
as a possible explanation. As possible causes of these biological 
cycles we shall examine self-induced effects within the population, 
random effects of many external factors, and single controlling in- 
fluences that themselves fluctuate in a random fashion or are truly 
cyclic. 

Regular oscillation in abundance of a single species might be pro- 
duced in a constant environment as the result of delays in the effects 
of excessive or favorable numbers. Such a population would alter- 
nately overshoot and undershoot a possible equilibrium level, as de- 
scribed in Chapter 9. When two or more species affect each other's 
abundance reciprocally, as may occur in prey-predator or host-parasite 
combinations, oscillations may be similarly set up, as pointed out in 
Chapter 10. Such oscillations caused by reactions within populations 
of one or more species are termed intrinsic cycles. 



Origin of Cycles 461 

As a species becomes more abundant, its metabolites tend to accu- 
mulate and other physical and chemical changes are brought about 
that may curtail the reproductive rate of the population. With in- 
creased density of the population predators are attracted and disease 
parasites can spread more easily. As crowding continues, food and 
other necessities become scarcer, and the animals tend to wander 
more extensively. This latter fact causes the further distribution of 
infectious disease and exposes the animals more frequently to detec- 
tion by their predatory enemies. Furthermore, with progressive re- 
duction of the vegetation by herbivores less cover for protection 
against their predators is available. Thus increase in abundance may 
automatically cause a curtailment of the population in ways that might 
tend to bring about regular self-induced oscillations. However, the 
inevitable occurrence of other changes in the environment would be 
expected to impose irregularities upon any intrinsic cycle. 

Approximately regular fluctuations in populations caused by in- 
dependent changes in the environment either animate or inanimate- 
are known as extrinsic cycles. Obviously intrinsic and extrinsic 
types of cycles are not sharply separated since many populations 
respond both to self-induced and independent variations in their en- 
vironment. Irregularities in the environment might set in motion 
regular internal oscillations by a sort of resonance, just as irregular 
waves can cause a boat to roll in its natural period. 

Scrutiny of the data on populations with apparently cyclic fluctua- 
tions has led Cole (1951) to conclude that "the oscillations of any 
hypothetical factor determining population size need only be about as 
regular as would be expected of a random variable." Since we know 
that the abundance of every species is affected by many environmental 
factors, it may be that chance combination of favorable and unfavor- 
able influences in any year will produce apparently cyclic fluctuations. 
Support for this simple explanation is found in a comparison of certain 
fluctuations in nature with those of a series of random sampling num- 
bers in which the former are no more convincingly "cyclic" than the 
latter (Fig. 12.15). Cole also urges that here, as elsewhere, the so- 
called "law of scientific parsimony" be applied, that is, the avoidance 
of a more complicated hypothesis than is actually required to explain 
the observed facts. However, it is unlikely that random fluctuation 
alone would produce cycles of the same period in widely different 
species. 

In some instances variation in abundance or growth has been defi- 
nitely related to the variation of some extrinsic factor in the environ- 
ment, and no doubt many more such relationships will be found in the 



462 



Succession and Fluctuation 



Tippett's numbers, 2-pomt moving sum 




1850 1860 1870 1880 1890 1900 1910 

FIG. 12.15. Fluctuating series of animal populations and tree growth compared 
with series of random numbers. In the 2-point moving sum curve the height of 
each point depends upon the values of two successive random numbers, much as 
population size might be affected not only by conditions this year but also by the 
size of last year's population. (Cole, 1951.) 



Origin of Cycles 463 

future. Fluctuation in populations of guano-producing birds is defi- 
nitely determined by periodic influx of warm currents. The growth 
of Douglas fir in Utah was found to be closely correlated with precipi- 
tation, but the growth of hemlock trees in Pennsylvania during the 
same years with the same average cycle period exhibited no significant 
relation to rainfall. The oscillations of the arctic fox ( Alopex lagopus) 
and of the snowy owl (Nyctea nyctea) are evidently timed by those 
of the lemmings and mouse-like rodents on which they feed. As the 
rodent population diminishes, the snowy owls disperse in search of 
food, and their periodic appearance as far south as New England 
attracts much interest among ornithologists (Gross, 1947). Maxima 
of the Canadian lynx follow those of the snowshoe rabbit by a year or 
so. The fact that the colored fox (Vulpes fulva) and the marten 
(Maries americana) exhibit a 4-year cycle in the arctic and a 9- or 
10-year cycle farther south is perhaps due to a corresponding differ- 
ence in the periodicity of its prey in the two regions. In at least some 
of these instances the fluctuations of the prey are not controlled by 
the predators as in reciprocating intrinsic cycles. For example, the 
snowshoe rabbits on Anticosti Island, where there are no lynx, fluctuate 
with the rabbit population on the mainland where lynx abound. 

In the foregoing type of situation the apparently cyclic fluctuation 
of the species is not due to a random effect of many environmental 
influences but is controlled principally by the changes in one par- 
ticular factor. The biological cycle might then be caused either by 
apparently cyclic random fluctuations of this controlling factor, or by 
a truly cyclic change in this factor. A long search has been made for 
a predictably regular cycle in the climate or in some other feature of 
the environment that might serve as a master timer for biological 
cycles, but no convincing and generally accepted factor has been 
found. Sunspots were at one time thought to be correlated with cer- 
tain population cycles since their average period is about 11 years and 
is thus closely similar to certain cycles of growth and abundance. 
However, the sunspot cycle is more variable than the Canadian lynx 
cycle, for example, since the period of the former has varied from 7 to 
17 years as against 8 to 12 years for the latter. In addition, a com- 
parison of the two cycles from 1750 to 1935 shows that, although they 
correspond for a number of years, they subsequently become com- 
pletely out of phase (Fig. 12.16). 

A satisfactory explanation of these intriguing cycles of growth and 
abundance accordingly continues to elude us. These fluctuations are 
another manifestation of the many changes going on within the com- 
munity that have been reviewed in this chapter. We have seen that 



464 



Succession and Fluctuation 




1750 1760 1770 1780 



1790 1800 1810 1820 
Time in years 



1830 1840 




1850 1860 1870 1880 1890 1900 1910 1920 1930 

Time in years 

FIG. 12.16. Population trends of the Canadian lynx (solid line) graphed against 
sunspot numbers (dotted line). (Alee et al., 1949 after MacLulich, 1937.) 

modifications brought about by the life processes of the inhabitants in 
their reciprocal relations with the environment cause ecological suc- 
cession to go forward from the initial colonization of a new biotope 
to the climax condition. The biomes of the world represent the per- 
manent establishment of the principal climax communities in general 
equilibrium with their environment. Within these generally main- 
tained conditions, however, many seemingly regular and many ir- 
regular fluctuations continually take place. 



13 

Dynamics of 
the Ecosystem 



In the two previous chapters we have discussed the composition of 
communities, their progressive changes, and the fluctuations that occur 
among their members. We shall now consider the dynamics of the 
ecosystem as a whole, that is, the operation of the community and its 
environment as a functional unit. We shall also discuss certain quan- 
titative aspects of the interdependencies in the ecosystem, including 
the concepts of productivity. 

FUNDAMENTAL OPERATION 

What natural situation shall we select for the study and illustration 
of the fundamental operation of the ecosystem? We might take the 
whole world as a unit. We could measure the energy received by 
the earth and follow its transformations; we could measure the 
amounts and kinds of organic matter that are elaborated. In other 
words, we could evaluate the earth as a mechanism for biological 
transformations. A study of this sort is of interest and involves sub- 
ject matter treated in the field of biogeochemistry in which the distri- 
bution and transformation of biologically important materials are 
traced (Hutchinson, 1948). However, we are more often concerned 
with the operation of particular types of ecosystems, and, in any event, 
an investigation of the variations from place to place and from time to 
time would be required before an adequate summation could be made 
for the entire world. 

In selecting an area for study we might go to the opposite extreme 
and investigate a single plant together with the space immediately 
surrounding it. Although certain relations of the exchange between 
the plant and its environment would be revealed, the unit studied 
would probably not be large enough to include all related influences. 
Ideally, it would be desirable to study an area that is large enough 
to be representative and to contain all the fundamental factors, and at 
the same time that is cut off from outside influences. A completely 

465 



466 Dynamics of the Ecosystem 

self-sufficient ecosystem rarely occurs in nature, but situations ap- 
proaching this condition may be found. A balanced aquarium 
represents an artificially established, self-contained system in which 
the number and type of plants and animals are adjusted so that a glass 
cover can be placed over the top and the organisms within will main- 
tain themselves indefinitely. A pond with no inlet or outlet, or with 
very little water exchange, may approach self-sufficiency. This idea 
was first set forth by Forbes ( 1887 ) in his classical essay, "The Lake 
as a Microcosm." In the following quotation it is clear that Forbes 
was thinking primarily of the animals in the lake and was not taking 
the complete ecosystem into consideration, but he states the essence 
of the concept of self-sufficiency, and his essay has become a landmark 
in the development of ecological thought: 

The animals of such a body of water are, as a whole, remarkably isolated 
closely related among themselves in all their interests, but so far independ- 
ent of the land about them that if every terrestrial animal were suddenly 
annihilated, it would doubtless be long before the general multitude of the 
inhabitants of the lake would feel the effects of this event in any important 
way. It is an islet of older, lower life in the midst of the higher, more re- 
cent life of the surrounding region. It forms a little world within itself 
a microcosm within which all the elemental forces are at work and the play of 
life goes on in full on so small a scale as to bring it easily within the 
mental grasp. 

If an isolated and self-sufficient microcosm cannot be found, a 
habitat may be used for study in which exchanges with other areas 
are slight and regular, or can be measured. Thus, the living and non- 
living materials entering a pond by way of the inlet and the substances 
being carried away by the outlet can sometimes be measured, and 
suitable allowance can be made. Another possibility is the study of 
a unit area, or unit volume, within a uniform region. The operation 
of ecological forces may be investigated in the ecosystem represented 
by 1 hectare in the middle of a large tract of forest, or in 1 sq km of 
grassland in range country, or in a certain area of the ocean. In these 
instances we know very well that the unit we select is not isolated from 
neighboring units, but we may assume that transfer out of our unit is 
compensated for by transfer into it from surrounding areas so that 
ecological activity within our area may be treated as if the study unit 
were self-contained. 



Principal Steps and Components 

The fundamental steps in the operation of the ecosystem are: (1) 
reception of energy; (2) production of organic material by producers; 



Principal Steps and Components 



467 



(3) consumption of this material by consumers and its further elabora- 
tion; (4) decomposition to inorganic compounds; and (5) transforma- 
tion to forms suitable for the nutrition of the producers. If the area 
is occupied by a self-sufficient community, all these steps will go on 
within it; otherwise materials must enter from surrounding areas. 
These steps in the operation of the ecosystem not only involve the 
production, growth, and death of the living components but also may 
influence the non-living aspects of the habitat. For example, con- 
siderable energy is represented in the organization of the soil that is 
brought about by the growth of the vegetation. 

The diagram in Fig. 13.1 illustrates the fundamental steps in a self- 



Essential components 



Non-essential components 



Herbivores [ [ Carnivores | 




Producers 



[Nutrients |< | Transformers 



Consumers 



Non-living Living 

components components 

FIG. 13.1. Principal steps and components in a self-sufficient ecosystem. 

sufficient ecosystem and shows the nature of the components taking 
part in the cycle of energy and material. The non-living components 
that must be present are light energy and inorganic nutrients for the 
growth of photosynthetic plants. The living components of the eco- 
system consist of producers and consumers. The producers are repre- 
sented primarily by green plants but also include synthetic bacteria. 
The activity of the synthetic bacteria is of minor consequence in most 



468 Dynamics of the Ecosystem 

instances, but in specialized situations it may be of quantitative im- 
portance. The consumers include all the other types of organisms in 
the community. Free-living herbivores feed directly upon the green 
plants. The principal herbivores in terrestrial habitats are insects, 
rodents, and ruminants, and in aquatic habitats they are small Crus- 
taceans and mollusks. Since most other prominent types of animal 
life in the community depend upon these herbivores, the latter have 
been referred to by Elton (1939) as "key industry animals." The 
herbivores serve as food for primary carnivores; these in turn may fall 
prey to secondary carnivores, and several more stages of nutritional 
dependency may exist. The tissues of the various plants and animals 
in the community may also be eaten by parasites, and after the organ- 
isms are dead, by scavengers and saprophytes of many sorts. The 
plants and animals that depend successively one on another form the 
links of a food chain. 

The dead bodies of the producers and of the consumers mentioned 
above are attacked by decomposers, consisting of bacteria and other 
types of fungi. The decomposers render the organic matter soluble 
and break it down chemically. The material is then attacked by 
transformers other types of bacteria that change the inorganic com- 
pounds into forms suitable to serve as nutrients for photosynthetic 
plants once again. Among the living components only the photosyn- 
thetic plant producers, the decomposers, and the transformers are 
essential. Some communities could theoretically maintain themselves 
indefinitely without the presence of any animals whatsoever, and, 
although it may injure our ego to state it, we realize that man is not a 
necessary part of the ecosystem. However, animal life is ordinarily 
present in most natural communities, and sometimes animals hold a 
controlling position or take a prominent part in the operation of the 
ecosystem. 

Niches. Different species of animals and plants fulfill different 
functions in the ecological complex. The role of each is spoken of as 
its niche. The term so used stresses the function of each organism 
in the community rather than its physical place in the habitat. The 
term originated, however, from the characteristic location of different 
types of organisms in the area under consideration, and niche is still 
used by some ecologists in this sense. The "functional niche" is more 
fundamental than the "place niche," but both concepts exist and 
should eventually be given different names*. 

Niche may be used in a broad sense to refer to the principal func- 
tions involved in the operation of the ecosystem, or it may be employed 
to describe the subdivisions of these and the various methods of 



Trophic Levels and Relations 469 

"making a living" within each functional category. Among herbivores 
that feed on trees, for example, some fill the niche of eating the leaves 
whereas others use the twigs, sap, bark, or roots as a source of food. 
In different geographical regions each type of niche is often filled by 
quite different species. Activity in some niches requires extreme and 
often bizarre specialization of anatomy, physiology, or behavior. 
Minute differences in function are exhibited by the niches of birds that 
obtain food organisms from crevices in bark. Small woodpeckers 
characteristically fly to the base of a tree and climb up the trunk 
looking for grubs. When they reach the top, they fly diagonally down- 
ward to the base of the next tree and again work their way upward. 
In contrast, nuthatches common in eastern United States ordinarily 
fly to the top of a tree and work toward the base, clinging upside 
down to the trunk. In this way woodpeckers readily remove grubs 
and other food from crevices easily reached from below, whereas the 
nuthatches obtain their prey from cracks in the bark more accessible 
from above. In the Galapagos Islands a finch that has evolved toward 
the woodpecker type fills this niche, but having no barbs on its tongue 
it wedges grubs out of crevices by means of a thin twig held in its 
beak. 

Trophic Levels and Relations. Each successive level of nourish- 
ment as represented by the links of the food chain is known as a 
trophic level. The plant producers within an ecosystem constitute 
the first trophic level, the herbivores form the second trophic level, 
and the primary carnivores represent the third level. Additional links 
in the main food chain, and in side chains such as those formed by 
parasites, constitute further trophic levels. Three types of pyramidal 
relations may be found among the organisms at different trophic levels 
in the ecosystem, resulting in the production pyramid, the pyramid of 
biomasses, and the pyramid of numbers. 

At each step in the food chain a loss of energy and of material from 
the system takes place because the processes of assimilation and 
growth are not 100 per cent efficient. This means that the organic 
matter produced per average unit of time, and the energy represented 
by it, become less at each successive trophic level. The production 
rates of the components of a self-sufficient ecosystem may thus be 
thought of as forming a pyramid. The base of this production 
pyramid is represented by the organic synthesis of the green plant 
component, and higher levels are represented by the growth rates of 
the herbivore and carnivore components. 

Frequently, the size, growth rate, and longevity of the species 
making up a particular ecosystem are such that the living weight, or 



470 Dynamics of the Ecosystem 

biomass, of the members of the food chain present at any one time 
also form a second type of pyramid. In a land biotope the biomass of 
the vegetation existing at the moment of observation is commonly the 
greatest, and the biomasses of herbivores, carnivores, and further 
Jinks in the food chain are progressively smaller. Thus a pyramid of 
biomasses results, but this pyramid is an indirect consequence of the 
particular kinds of organisms present, in contrast to the production 
pyramid which is a dynamic necessity in the operation of the eco- 
system. In some aquatic biotopes the biomasses of some of the in- 
habitants form a pyramid similar to the production pyramid, as in 
Weber Lake, Wisconsin (Fig. 13.2), and on Georges Bank (Fig. 
13.12), but in others the biomasses of fish and particularly of the 




Bottom flora 



Phytoplankton 



FIG. 13.2. Pyramid of biomasses as illustrated by the weight relationships of the 
various constituents of Weber Lake, Wisconsin. (Juday, 1943.) 

bottom fauna are considerably greater than those of shorter-lived 
components at lower trophic levels (Harvey, 1950, Table IV). The 
operation of the ecosystem thus limits the relative amounts of the 
components that may be produced, but departures from a regular 
reduction in the biomasses of components present at any one time 
occur because of special circumstances. 

Interdependencies within the ecosystem also exert a certain influ- 
ence on the size of the individual organisms involved. The essential 
plant producers vary in size from the very largest organisms in some 
types of community to the smallest in others. The primary producers 
in different biocenoses range from giant trees, such as the redwoods, 



Trophic Levels and Relations 471 

to diatoms, often less than 30 //. in diameter, and green flagellates or 
photosynthetic bacteria of still smaller dimensions. 

The possible size of consumers is generally influenced by the size of 
their food, but the possession of specialized food-getting equipment 
enables some consumers to be very different in size from the organism 
that provides their nutriment. Herbivores may be either larger or 
smaller than the vegetation upon which they feed. Insects inhabiting 
forests are characteristically only a minute fraction of the size of their 
food plants, Some rodents are larger and some smaller than their 
food; crustaceans and mollusks that filter phytoplankton from natural 
waters are generally very much larger than the plants upon which 
they feed. 

In the next step in the food chain, namely the subsistence of the 
carnivores on the herbivores, the predatory species must be stronger 
in some respect than its prey and is usually larger. Among animals, 
therefore, a tendency exists for smaller species to form the early links 
in the food chain and for larger species to form the later links. The 
maximum size of a prey species that can be attacked successfully de- 
pends upon the feeding apparatus of the predator. Thus some or- 
ganisms in the community rnay be too large and some may be too 
small for the consumer to catch or to obtain in sufficient quantity. 

Since total biomass tends to become smaller at successive levels in 
the food chain, and since size of individual generally becomes larger, 
at least among the animals, it follows that in the typical situation a 
reduction in the number of individuals comprising successive links 
in the food chain takes place. A relation of numbers is another con- 
sequence of the operation of the, ecological complex. Among the ani- 
mals of a community the herbivores are typically the most numerous; 
they take in food material synthesized by the plant producers and 
pass it on to the subsequent consumers. Primary carnivores that prey 
upon these "key industry animals" are less abundant, and secondary 
and tertiary carnivores generally exist in still fewer numbers. This 
numerical relationship with the more abundant species near the base 
of the food chain and the less abundant species near the top is known 
as the pyramid of numbers. In food chains involving parasites the 
size relationships are reversed because the parasite is smaller than its 
host, and hyperparasites must be still smaller. For this reason the 
pyramid of numbers is reversed for the successive steps of parasite 
dependency, and the parasites of each link are generally more numer- 
ous than their hosts. *^ 

An illustration of the pyramid of numbers among mammals and 
birds inhabiting range land is presented in Table 20. The differences 



472 



Dynamics of the Ecosystem 

TABLE 20 



ABUNDANCE OF BREEDING ANIMALS ON 1 SQUARE MILE (3 sq km), SANTA RITA 
RANGE RESERVE, ARIZONA (After Leopold, 1933) 

Species Number 

Coyote 1 

Horned owl 2 

Redtail hawk 2 

Blacktail jackrabbit 10 

Hognosed and spotted skunk 15 

Roadrunner 20 

Cattle (over 1 year old) 25 

Scaled quail 25 

Cottontail 25 

Allen's jackrabbit 45 

Gambel quail 75 

Kangaroo rat (Dipodomys) 1,300 

Wood rat (Neotoma) 6,400 

Mice and other rodents 18,000 

in abundance of the carnivores, such as the coyotes, hawks, or owls, 
and of the herbivores, such as the quail or the mice, are striking; an 
even greater contrast in numbers would have appeared if the census 
had included the insects and other invertebrates. An example of the 
pyramid of numbers on a small scale is furnished by a census of the 
metazoans inhabiting the floor of a deciduous forest (Fig. 13.3). 
In addition to the foregoing relations of biomass, size, and num- 



16-20 

E ! 

lM5 



6-10 



1-5 



<1 



Number per kilogram 

FIG. 13.3. Pyramid of numbers among the metazoans of the forest floor in a 
deciduous forest near Evanston, Illinois. (Park, Alice, & Shelford, 1939, Copy- 
right, Univ. of Chicago Press.) 




Ecological Cycle in the Ocean 473 

bers, certain aspects of availability are of crucial concern in the eco- 
system. Food organisms must be sufficiently concentrated in relation 
to their size so that the predator can meet its nutritive requirement 
within the time available. In other words, the abundance of the 
food per unit area must be adequate in relation to the possible forag- 
ing range of the animal concerned.] If there were only one lion in 
all of Africa and zebras lived 100 miles apart, plenty of food would 
obviously exist for the one lion, but he would not be able to obtain 
it fast enough to satisfy his needs. The same general point is illus- 
trated by the facetious remark that in parts of Texas grass is so scarce 
that the cattle must feed on the run. The necessity for adequate con- 
centration as well as for adequate total amount of food is not so 
immediately apparent for organisms in which feeding processes are 
not directly observed. The total amount of planktonic food in an 
oceanic area may be more than sufficient for the nutritive needs of 
the clams or oysters living on the bottom, but in order for these 
bivalves to survive the food must be concentrated to such an extent 
that the volume of water that they can filter in a day will furnish 
enough plankton to fill their daily needs. 

The necessity for an adequate rate of replacement of nutritive 
materials is another aspect of availability in the food relations of a 
community. The growth of terrestrial plants is dependent not only 
upon the concentration of nutrient salts in the soil at the moment of 
observation but also upon the replenishment of these nutrients after 
the existing supply has been absorbed. The same dynamic de- 
pendency exists in all natural situations where the nutrition of one 
step in the food chain depends upon the rate of supply by the previous 
step. In many situations the amount of nutriment supplied per unit 
of time is larger than the amount of the material present at any one 
moment. The same argument applies to the dependency of a car- 
nivore on a prey species. Whether or not the food organism is 
adequate in amount depends both on its abundance at the moment 
and also upon the rate of replacement of the population. 

Ecological Cycle in the Ocean 

The ecological cycle in temperate regions of the ocean will be used 
to illustrate the principles involved in the general operation of the 
ecosystem as well as many of the relationships discussed in earlier 
chapters. The open sea is especially suitable for this purpose since 
a sampling can be made in the middle of a large uniform area with 
fair assurance that no serious modification by interchange of materials 



474 Dynamics of the Ecosystem 

from surrounding areas of a different sort will take place. As has 
been pointed out in earlier chapters, the ocean represents a relatively 
uncomplicated biotope compared to many others, particularly those 
in the terrestrial environment. In a volume of sea water it is possible 
to measure with accuracy the physical and chemical factors with 
which we are concerned and to enumerate the living components with 
sufficient precision. 

Producers. In the open sea the planktonic plants are the essential 
producers since the reduction in light with depth prevents benthic 
plants from growing in important numbers in deep water. The main 
bulk of the phytoplankton is represented by diatoms, but green flagel- 
lates and other types of nannoplankton may also play a significant 
role because of their great numbers and the rapidity of their repro- 
duction. The preponderance of unicellular plants in the oceanic com- 
munity is related to the retardation of sinking rate resulting from small 
size and to the increased effectiveness with which widely distributed 
cells can absorb dilute nutrients. The zones of diatom growth are 
sometimes spoken of as the pastures of the sea, and the phrase current 
on land, "All flesh is grass/' has been paraphrased for the ocean as 
"All fish is diatoms," 

Since diatoms and other types of phytoplankton represent the pro- 
ducer component of the oceanic ecosystem, we shall consider briefly 
their typical cycle of abundance and the factors controlling it. In the 
temperate ocean a sudden outburst of diatom growth generally occurs 
in the spring months (Fig. 13.4), and this is often called the "spring 




Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 
FIG. 13.4. Generalized diagram of seasonal cycle of diatom abundance and cer- 
tain controlling factors in the temperate ocean. 



Producers 475 

flowering" or "bloom"an amusing misnomer since these algae are far 
removed taxonomically from flowering plants. In the Gulf of Maine, 
for example, the number of diatoms per unit volume in the spring 
averages 1000 times greater than the winter population and may be 
over 60,000 times greater in local regions. Hardly has the diatom 
population reached its maximum size in March or April than numbers 
begin to drop off and characteristically continue to fall to a low level. 

An intriguing interplay of ecological forces accounts for these 
enormous changes in the numbers of plant producers. The thorough 
stirring of the water in the upper layers of the ocean during the 
winter brings about a replenishment of the nutrients to the euphotic 
zone from the deeper levels, but the same vertical movement of water 
also carries the diatoms down to levels below the compensation 
depth. As the spring comes on, two things happen: the compensation 
depth extends to deeper levels because of the greater intensity of sun- 
light, and stirring becomes less because of the progressive stratifica- 
tion of the water. As a result the diatoms remain for longer and 
longer periods above the compensation depth, until after a time con- 
structive growth of the population becomes possible (Sverdrup, 
1953). Under favorable conditions diatoms can divide at a rate 
greater than once in 24 hours. Exponential growth of the population 
ensues and the sea may become green with diatoms within a week. 

Certain definite ecological reactions bring the spring increase in 
phytoplankton to an end. As diatom growth goes forward, nutrients 
are progressively used up until their concentration drops to a point 
where all the plant cells are starved and either die or form cysts. 
Furthermore, the great abundance of diatoms has tended to reduce 
the transparency of the water and at the same time has provided a food 
supply for a new generation of zooplankton. Copepods and other 
herbivores "graze" on the diatoms, arid, as the zooplankton increases 
in abundance, the diatom population is consumed at an accelerating 
rate. 

The diatoms of the open sea characteristically remain at a low 
ebb throughout the summer months because the majority of dead 
cells decompose and release nutrients again at levels below the com- 
pensation depth. Because thermal stratification prevents deep stir- 
ring, the euphotic zone remains depleted of its nutrients during the 
summer. In the autumn when stronger winds and lower surface tem- 
peratures allow effective stirring to take place once again, nutrients 
are restored to the upper layers of the ocean. A rather sudden in- 
crease in the diatom population may then take place; this is known 
as the autumn "flowering" or "bloom." As the diatoms again grow 



476 Dynamics of the Ecosystem 

in abundance, they reduce the nutrient supply concomitantly for a 
time. Following the subsequent complete breakdown of thermal 
stratification, wind stirring and reduced illumination stop diatom 
growth as the area returns to its winter condition once more. How- 
ever, the same deep stirring during the winter brings the nutrient- 
rich water to the surface. This annual restoration of deep water to 
the upper layers gives rise to the statement that "the sea is plowed 
once a year." 

Consumers. When we turn to a consideration of the consumer in 
the typical food chain of the open sea, we are confronted with a 
situation of special interest because of the nature of the producers. 
The simple fact that the plants at the base of the food chain are micro- 
scopic in size has far-reaching repercussions in the oceanic ecosystem. 

(a) Herbivores. Scarcely any of the larger marine animals are 
able to feed directly on diatoms because of their small size. The 
menhaden is one of the few fish of commercial importance that can 
do so. The gill rakers of this species are usually long and provided 
with interlocking hooks that form a fine, sieve-like structure by means 
of which the plant cells can be filtered from the sea in sufficient 
quantities. The production of the menhaden thus represents a two- 
link food chainthe shortest possible and a rarity among the large 
animals of the sea in complete contrast to the situation on land. In 
the terrestrial environment the availability in respect to size and 
abundance of trees, grass, and other plant producers is such that deer, 
elk, sheep, and other large ruminants can feed directly on them. Thus 
short food chains are common on land. 

Since most of the larger animals in the sea are unable to use phyto- 
plankton as food, they must depend for their nutriment upon "middle 
men" or "key industry animals" of intermediate size. On the sea 
bottom many mollusks and other sessile invertebrates possess feeding 
mechanisms that enable them to filter the smaller plankton ic forms 
from the water. In the open sea planktonic crustaceans, especially 
copepods and euphausiids, fill this niche. Copepods can filter out the 
smallest diatoms by forcing a current of water through a meshwork of 
bristles attached to a special set of appendages. We might think of 
these small crustaceans acting as miniature harvesting machines, 
packaging the microscopic plant cells. The filter-feeding zooplankton 
incorporate the substance of the diatoms and represent parcels of 
food of sufficient size for larger animals (Fig. 13.5). 

When the importance of copepods as essential middlemen in the 
economy of the sea was recognized, special attention was directed 
toward the factors controlling the food supply of these animals. 



Consumers 



477 



However, food dependencies are difficult to determine, and several 
divergent views have been held as to the critical relations. Since 
observations at sea indicated that diatoms were often abundant when 
copepods were scarce and vice versa, some investigators argued that 
no direct food dependency occurred. The quandary of this apparent 
contradiction represents the danger of generalizing from isolated ob- 
servations or from statistical tabulations of the occurrence of animals 
and plants without due regard to the dynamic aspect of their growth 
and life cycles. If diatoms are scarce at a given point where 
copepods are abundant, does it mean that the copepods have just 
finished consuming the population, or does it mean that copepods are 
primarily dependent upon some other source of food? 




Clarke. 1943 

FIG, 13,5. Simplified representation of main ecological cycle in the sea. 

As data on this problem were gathered, two theories emerged. 
One was the theory of animal exclusion proposed by Hardy (1936) 
that high concentrations of phytoplankton are harmful to zooplankton 
and, therefore, that water masses containing large numbers of diatoms 
are actively avoided by copepods. Exclusion was originally believed 
to have taken place chiefly in the vertical direction and thus to curtail 
the upward phase of the diurnal migration of the zooplankton. Hori- 



478 Dynamics of the Ecosystem 

zontal currents moving differentially at various levels would secondar- 
ily bring about a patchiness in horizontal distribution. Subse- 
quently, the scope of the theory has been extended to include other 
types of exclusion of one population by another. Dense diatom con- 
centrations might be inimical to copepods either mechanically or 
chemicallypossibly through antibiotic effects. Such effects have 
been demonstrated for oysters and cladocerans in certain instances 
as described in Chapter 8. 

At about the same time, Harvey and co-workers ( 1935 ) came for- 
ward with the theory of grazing, according to which copepods are 
believed to reduce the diatom population locally by the intensity of 
their feeding activity. At first sight these theories seemed mutually 
exclusive, but more extensive investigation has shown that the in- 
fluences involved in both may be in operation simultaneously without 
conflict. Copepods depend upon suitable abundance of diatoms 
for food, but they may avoid thick concentrations of diatoms and, by 
grazing around the margins of diatom populations, may accentuate 
the patchiness in the distribution of the latter. The timing of the 
supply of diatom food in relation to the appearance of new broods of 
copepods is also critical (Clarke, 1939). Investigation of the pro- 
duction of Calanus finmarchicus in the Clyde Sea area gave evidence 
that those broods of this copepod which hatched during periods of 
diatom scarcity failed to reach maturity, whereas broods appearing 
when suitable diatom food was available developed successfully 
( Marshall